Time ' s Arrow ( episode ) An engineering team finds evidence of an alien presence on Earth in 19th century San Francisco : Data ' s severed head , buried five hundred years ago . ( " Season finale " ) Summary . Teaser . Captain Picard and Lieutenant Commander Data enter an underground cavern in San Francisco with a scientist where work crews have discovered some artifacts dating to the 19th century . Data notes the presence of a " bifocal visual aid " that is typical for the era and a revolver which is identified as a " . 45 caliber double action cavalry pistol invented by Colt Firearms in 1873 . " A gold watch is also present with the inscription " To S . L . C . With Love 30 November 1889 . " The time on the watch is 9 : 39 : 46 . The scientist leading them explains that the work crews installing seismic regulator s in the cavern under San Francisco were having difficulty with the phase conditioner s due to the composition of the walls of the cavern . Data scans the rock walls with a tricorder and determines that the rock has been altered by exposure to triolic wave s which are foreign to Earth in both the 19th and the 24th centuries . Picard wonders aloud why the " Enterprise " would be called back for such a discovery when there are experts on Earth better suited for an investigation . The scientist reveals that there is one more item they found . He lifts a cover to reveal Data ' s severed head . Act One . Data and Commander Riker are in engineering examining Data ' s head . Data notes that there is a twelve percent decomposition of bitanium in the neural pathway links . Riker is obviously uncomfortable and snaps at Data when he refers to his head as an artifact . Picard arrives and asks if the head could be that of Data ' s brother , Lore . Data says no because his brother ' s positronic brain has a Type L phase discriminating amplifier while his is a Type R . He estimates that the head has been in the cavern for approximately five hundred years . He further asserts that at some future date , he will be transported back in time to Earth , where he will die . Riker says they will do everything they can to prevent it , but Data appears to accept his death as inevitable - " " It has occurred . It " will " occur . " " Lieutenant Commander Geordi La Forge interjects that he has some information on who the aliens in the cavern might have been . He believes they are dealing with a species with microcentrum cell membrane s because triolic waves wouldn ' t harm them . Further , they may be shapeshifter s . La Forge also discovers a cellular fossil , specifically , a microscopic ciliated lifeform designated LB10445 , only known to exist on Devidia II in the Marrab sector . Picard orders Riker to lay in a course for Devidia II . Later , in Ten Forward , La Forge and Data are discussing Data ' s death . La Forge posits that the discovery must bother Data a little , but , to the contrary , Data finds it comforting . Previously , Data believed that he may exist for an unlimited period of time . This reinforced the fact that he was artificial and even more different from Human s who age . He explains that he is now more like everyone else because he is mortal . More importantly , prior to the discovery of his mortality , he expected to outlive everyone he considers his friends , and make new ones which he would then outlive as well . Mortality makes his existence complete in a way immortality cannot . He thus feels that he can " look forward " to death , since he has reviewed his existence to date and can be satisfied that he has accomplished a great deal . Picard ' s voice over the intercom interrupts them . He orders all bridge officer s to their stations as they are approaching the Devidia system . Data leaves and Guinan asks La Forge about their conversation . He tells her that they found Data ' s head and it had been there for five hundred years . Guinan says , " " Full circle . Hmm . . . " " as La Forge leaves for engineering . Counselor Troi and Riker are walking in a corridor on deck 12 toward the turbolift discussing the recent events . Riker expresses that he is angry about the situation . Troi says it is like finding out that someone you love has a terminal illness . Just then the turbolift doors open and Data is inside . They immediately cease their discussion and enter the lift . Data looks sideways at both of them before telling them that people are behaving differently around him , such as by abruptly ending conversations as they just did when the doors opened . Troi says that he ' s right and that it wasn ' t a very nice thing to do . Riker explains , " " It ' s just that our mental pathways have become accustomed to your sensory input pattern . " " Data says he understands and that he ' s fond of them also . They exit the lift onto the bridge . The " Enterprise " is in orbit of Devidia II . Picard enters the bridge from his ready room and asks if there are any signs of life . Lieutenant Worf answers in the negative . Data announces that sensor s have detected a temporal disturbance on the planet ' s surface at 42 degrees , seven minutes north by 88 degrees declination east . A spectral analysis shows a high level of triolic waves at the same location . The magnetic signature of the waves is identical to those found in the cavern under San Francisco . Picard asks if the waves are dangerous and Data replies " " only with long - term exposure . " " Picard then orders Riker to take an away team to the surface . Riker orders Worf , La Forge and Troi to join him in transporter room three . Data is left out as Picard explains to him that he is needed to monitor sensor readings . Data asks to speak with the captain alone and they move the conversation into Picard ' s ready room . Once alone , Data notes that standard procedure calls for the second officer to accompany away teams . Picard explains that he is just being cautious . Data observes that there is not rational justification for the decision to which Picard snaps , " " Then I ' ll be irrational ! " " Data then employs an aphorism saying , " " One cannot cheat fate . " " Picard replies , " " Perhaps we can ' t , Mr . Data , but at least we can give it a try . " " The away team materializes in a cavern under the surface of the planet . La Forge scans for and finds triolic waves but can ' t determine their source . He contacts Data and asks him to run a spectral field correlation to see if the waves are related to the temporal distortions . Troi is standing in the middle of a large room and says , " " There ' s life here . " " She senses hundreds of lives . They ' re all terrified and they ' re all Human . Act Two . Riker reports to Picard from the surface that Troi is convinced that there are Humans present and that they may be trapped . Data reports that his temporal analysis indicates a synchronic distortion in the areas emanating triolic waves . The displacement is a positive . 004 percent which is enough to render the Devidians invisible due to being out of phase with the away team . La Forge hypothesizes that if they could create a contained subspace force field , they may be able to compensate for the phase variance , however , a very sensitive phase discriminator would be needed . Data allows that he has one built into his positronic decompiler and that he will need to join the away team to continue the investigation . Picard tells him to proceed . Data transports to the planet with a device for adjusting the phase of the subspace force field . He explains to the away team that he will become invisible after he adjusts his phase but that they will still be able to hear him because his combadge will transmit on a delay . The communication will not be two - way , however . As Data adjusts his device , he begins to disappear . He completely vanishes when he reaches . 004 . He reports visual contact with lifeforms in the room but they don ' t react to his presence . He describes vaguely humanoid creatures , two to three meters in height , silver - grey in color with an orifice in their foreheads . They are ingesting some type of energy fragments . There is a container with perhaps thousands of these fragments in the center of the room . As Data moves northward , he encounters an ophidian held captive in a force field . The away team can only stand in place and listen as his communication becomes more obscured by static with each passing moment . Data describes to them that two Devidians approach the ophidian and release it . A loud noise interrupts the communication and Data reports through much interference that he is caught in the aftereffect of a temporal distortion that was opened by the creatures . A bright flash of light is seen in the cavern and the phasing device that Data was carrying falls to the floor . Troi yells Data ' s name and Riker reports to Picard that they have lost him . Data finds himself lying on his back in the middle of a brick street outside fire house # 5 as a horse - drawn buggy goes by . He is in 19th century San Francisco . Act Three . Data is walking around the streets of San Francisco asking about two individuals with an ophidian . He gets ignored , laughed at and even called a French man by the people . He finds a copy of the " San Francisco Register " dated Sunday , August 13 , 1893 with the headline , " Cholera Outbreak . " A forty - niner asks for his help . Data explains that he has no money but is in search of information . The forty - niner tells him some basic rules of panhandling . Data walks up to a bellboy outside the Hotel Brian and asks for temporary lodging and is told the cost is " six bits a day or four dollar s a week . " Data explains that he has no money but that he can work . The bellboy says he himself already performs all of the work and walks away past a sad - looking man leaving the hotel . The bellboy remarks to Data that the man hasn ' t filled a straight in five weeks . Data looks at the bellboy and asks simply , " " Poker ? " " Data enters a smoke - filled room where four men , a seaman , an American Indian in a hat , a well - dressed man and another man , are playing poker . He asks to join the game and remarks ( to explain his appearance ) that he is a Frenchman . The well - dressed man speaks to him in French with a Louisiana accent . Data replies in French and is invited to join them . He sits down and offers his combadge in exchange for currency . He explains that it is a crystalline composite of silicon , beryllium , carbon - 70 and gold . The well - dressed man offers him three dollars for it . Data accepts and deals the cards . Data walks into his room at the Hotel Brian wearing the Indian ' s hat and the well - dressed gentleman ' s vest . The bellboy is talking about how he beat the card sharks , Frederick La Rouque and Joe Falling Hawk . He gives Data the key to the room and holds out his hand with the palm upturned . Data shakes his hand . The bellboy clears his throat and puts his hand out again for a tip . Data advises him to monitor his cough as there is a cholera epidemic in San Francisco . He clears his throat again and Data finally understands . He hands a dollar to the bellboy , who is very impressed with the amount of his tip and offers to get Data whatever he needs . Data writes a list with his left hand and gives it to the bellboy . Data explains the odd list of items by saying that he is an inventor . He then gives the bellboy a large sum of money and tells him to " " retain the surplus for yourself . " " The bellboy leaves the hotel in search of the items on the list and passes the old forty - niner Data had encountered before still begging on the street . A tall woman and a gentleman approach him . The man has a walking stick with the head of a snake on it . The woman lifts her leather bag and a blue beam of light emanates from the end and strikes the old man in the chest . He goes pale and dies . The beam is stopped and the two turn and walk away arm in arm . Act Four . The senior staff is assembled in the observation lounge . They are speculating on the disappearance of Data and the purpose of the Devidian ' s travels to earth . La Forge says that he may be able to reproduce what Data did to reach the Devidian ' s phase , but it would be difficult and take quite some time . Picard doesn ' t want anyone else going alone and La Forge says he can make a large enough subspace field to encompass everyone . Picard says , " " Make it so . " " Worf takes Riker aside as the rest of the senior staff exit the lounge after Riker suggests going after Data in the past . Worf notes that it may be their fate to die in the cavern five hundred years before with Data as their remains would have turned to dust long ago . In Ten Forward , Guinan is mixing drinks when Picard enters . She tells him she is mixing a Tzartak aperitif and that it is a very , very tricky recipe as it can easily just evaporate . She asks if Picard is going along on the away mission back to the surface . He says it is general policy not to . She asks him if he remembers the first time they met . When he says yes , she replies , " Don ' t be so sure . " She adds that he must go on this away mission or they will never meet . Data has assembled many components into a device on a table in his room . The bellboy enters with an anvil , which Data explains he needs for a low intensity electromagnetic field core . Data almost exposes his android abilities by thoughtlessly lifting the anvil in one hand . However , he corrects the mistake by pretending to have overexerted himself and stretched a muscle . The bellboy , whom Data calls " Jack " leaves him a croissant wrapped in a page of an old newspaper . Data sets the pastry aside , looks at the paper and sees a picture of Guinan under the headline , " Literary Reception Announced . " The date on the paper is Friday , August 11 , 1893 . Act Five . At the literary reception , Samuel Clemens , while lighting a cigar , is holding forth about Alfred Russel Wallace and his theories about Earth being the only life - supporting planet in the universe while Guinan , dressed in period clothing , listens and comments on his words . She posits that perhaps Earth is one of millions of inhabitable planets in the heavens . Clemens agrees and notes how insignificant mankind is in the grand scheme of things . Data enters the house and is stopped at the door and asked his name . He is not on the guest list and is asked to leave immediately . He dodges the doorman , enters the reception and calls out to Guinan . When he finds her he asks to be able to speak with her . She doesn ' t know him so he tells her they were on a ship together – a starship . Immediately she feigns knowing him and excuses herself and Data from Clemens ' presence . Outside , Guinan asks Data if her father sent him . He says no and explains their service together on the " Enterprise " . He tells her how he came to be in the 19th century and that he assumed that she had followed him from the 24th century . Noting her reaction he indicates that although he knew that her species was so long lived he had no idea that she had visited Earth so long ago . A wisp of smoke drifts by giving away the presence of Clemens standing in the open doorway listening to their conversation . La Forge , Troi , Worf , Riker and Doctor Crusher are in the cavern on Devidia II setting up equipment when Picard beams down . La Forge establishes the subspace field and Picard orders Worf to return to the " Enterprise " . La Forge adjusts the synchronic distortion and as they get nearer to . 004 , the Devidians flicker into view . Luminescent spheres exit a container in the center of the room one by one , then swirl around before entering an orifice on the foreheads of the aliens . Crusher scans them with a tricorder and says the strands may be organic . Troi says there is no life , but she senses an echo of the last moment of Human life . " " They all died in terror . " " A bright flash of light and a loud noise make the away team turn to see an opening appear in the wall of the cavern . Two Devidians enter , one holding a staff with a snake ' s head on it . They deliver more energy fragments and go back toward the temporal door . With grim determination , Picard leads the " Enterprise " crew into the vortex and into the unknown danger that awaits them . Memorable quotes . " " At some future date , I will be transported back to 19th century Earth , where I will die . It has occurred . It " will " occur . " " " " However , this investigation began with your death . I ' m trying to see it doesn ' t end that way . " " " " How did he put it . . . " ( impersonating Data ) " ' As I experience certain sensory input patterns my mental pathways become accustomed to them . The inputs eventually are anticipated and even missed when absent . ' " " " " It ' s just that ' our mental pathways have become accustomed to your sensory input patterns . " ' " " " Ah . I believe I understand . I am fond of you as well , Commander . And you too , Counselor . " " " " Your head is not an artifact ! " " " " So . . . do you want to talk about it ? " " " " Are you referring to the foreknowledge of my death ? " " " " I never knew how tough this must be for you . " " " " Tough ? As in difficult ? " " " " Knowing that you would outlive all your friends . " " " " I expected to make new friends . " " " " True . " " " " And then to outlive them as well . " " " " It provides a sense of completion to my future . In a way , I am not that different from anyone else . I can now look forward to death . " " " " I am presently in need of information . " " " " The stockbrokers are cheap as hell – don ' t even ask ' em . Best handout is a young man with his lady . You give him a chance to impress her by bein ' generous . But stay clear of the sailors – likely as not , you ' ll get a fist across the jaw for your trouble . " " " " Thank you for the advice , but I am trying to find two individuals with a snake . " " " " Pale face . " " " " Well , I don ' t like easterners , personally . " " " " I am a Frenchman . " " " " Mes parents sont originaires de Bourgogne . Je suis né à la New Orléans . " " ( Translation : " " My parents come from Burgundy . I was born in New Orleans . " " ) " " Alors , nous sommes presque frères . Je suis heureux de vous connaître . " " ( Translation : " " We are almost brothers , then . I am happy to know you . " " ) " " Looks like the missus booted you out in the middle of the night ! " " " " Ah . I understand the source of your misperception , but this is not sleepwear , and I do not have a ' missus . ' " " " " Your suspicions , Madame Guinan , are undoubtedly based upon your keen observational skills . Ha , ha . Now , if you ' ll permit me , I ' ll continue my character assassination unimpeded . " " " " My dear Mr . Clemens , please do . Please do . " " " " Full circle . Hmm . . . " " " " Isn ' t that what makes America great ? " " " " To what are you referring ? " " " " A man rides into town in his pajamas , wins a grubstake at a poker table , turns it into a horseless carriage and makes a million bucks . That ' s America ! " " " " I believe I have given you an erroneous impression . " " " " Data , is this yours ? " " " " My God Will , they ' re Human . " " " " Do you remember the first time we met ? " " " " Of course . " " " " Don ' t be so sure . I just mean . . . if you don ' t go on this mission , we ' ll never meet . " " " " Eavesdropping is by no means a proper activity for a gentleman ; never - the - less , the deed is done ! " " Links and references . References . 1873 ; 1893 ; 49er ; anvil ; apple ; bakery ; banana ; barber ; bifocal visual aid ; ; bitanium ; borax ; Burgundy ; byproduct ; cameo ; candle ; cannery ; card shark ; ; Chapman ' s Ice Cream ; cholera ; cigar ; clipper ship ; Coahuila ; Colt . 45 ; Colt Firearms ; communicator ; cough ; cranial unit ; decomposition ; Depew ; Devidia II ; Devidia system ; Devidian ; diamond ; dime ; diving horse ; Dr . Haile ' s ' Old Injun ' System Tonic ; driver ; drug broker ; Earth ; Easterner ; evolution ; exochemistry ; extraterrestrial ; fossil ; French language ; fruit ; Gentleman Jim ; geology ; Germany ; guest list ; handshake ; horse ; hotel ; Hotel Brian ; inventor ; J . Mees Rodent Control ; Kingsbury Pale ; La Forestal FCL ; La Rouque ' s parents ; LB10445 ; Lillian ; liniment ; literary reception ; Lore ; Luna ; maid ; Manitowoc ; Manitowoc Products Co . ; Marrab sector ; match ; microcentrum cell membrane ; " " ; mile ; Murphy Harmonika ; Native American ; New Orleans ; Number one ; No . 5 Fire House ; occupation ( job ) ; Ole Injun ; orange ; ophidian ; oyster ; oyster pirate ; Pacific Coast ; painting ; phase conditioner ; phase decompiler ; phase discriminator ; phase variance ; pistol ; prairie ; Putney ; saloon ; Saltillo ; " San Francisco Register " ; Sector 001 ; Starfleet Academy ; stockbroker ; Third ; tip ; tricorder ; turbolift ; ; Tzartak aperitif ; whiskey ; Wisconsin