Operation - - Annihilate ! ( episode ) The Deneva colony is attacked by neural parasites that cause mass insanity while the crew of " Enterprise " search for a way to stop them . ( " Season finale " ) Summary . Teaser . The approaches Deneva . Captain Kirk is concerned ; Uhura has been unable to contact any transmitter on the planet , including a private transmitter code Kirk suggests . Spock ' s research has revealed that a pattern of mass insanity has been spreading in a straight line through this part of the galaxy , starting with Levinius V , then Theta Cygni XII , Ingraham B , and now Deneva is next , a planet on which Kirk ' s brother Sam is stationed with his family . Lieutenant Sulu picks up a ship on sensor s . The small craft of Denevan configuration is on course directly for the , and does not appear to be out of control . Kirk orders Sulu engage the " Enterprise " at warp 8 interception course . The Denevan vessel is out of range of the tractor beam ; the " Enterprise " pursues . Finally , they make contact : seconds before the ship burns up , the pilot cries out " " I did it ! It ' s finally gone ! I ' m free ! ! " " . After the vessel ' s destruction , Doctor McCoy walks up to Kirk and softly asks , " " Jim , your brother Sam and his family – aren ' t they stationed on this planet ? " " Act One . Spock and Montgomery Scott inform Kirk about Deneva ' s history and the fact that there has not been contact in a year . Uhura reports activity on the private transmitter channel , a woman speaking but does not respond to Kirk . He identifies her as his brother ' s wife to McCoy . Once the " Enterprise " arrives at Deneva , Kirk forms a landing party consisting of him , Spock , McCoy , Scott , Yeoman Zahra , and Bobby . Spock says there are the expected number of Human life signs , but " strangely quiet . " They beam directly into the capital city on Deneva . Once on the planet , they are struck by the curious lack of people ; in a city of 100 , 000 people , no one is visible – until , a few minutes later , they are about to be attacked by four men who , even as they charge , scream " " Go away ! We don ' t want to hurt you ! " " But , with their crude clubs , they start to try anyway , forcing the landing party to stun them with their phaser s , an attitude inconsistent with their actions . Then McCoy discovers with his medical tricorder that the nervous system s of the unconscious men are violently active – as if they are somehow still being stimulated . A loud scream draws them next to Kirk ' s brother ' s lab . Kirk ' s brother , Sam , lies dead on the floor . Aurelan , Sam ' s wife , is hysterical , and their child Peter is unconscious nearby . Evidence suggests something has been trying to force its way in , despite the fact that the sensors showed nothing on Deneva that didn ' t belong there . In sickbay aboard the " Enterprise " , Aurelan , in terrible pain , tells Kirk that " things " came , eight months prior , on a ship from Ingraham B . As she tries to answer Kirk ' s questions , she experiences more and more pain , until McCoy is forced to sedate her . The creatures use the Denevans as their arms and legs , and are forcing them to build ships . They control their hosts with pain . Aurelan ' s last act is to implore Kirk not to let the things go any further ; this effort costs her everything she has left , and she dies . Kirk rejoins the landing party ; he knows there is some sort of creature present , but the landing party has not yet discovered anything beyond a curious buzzing . Entering a building where they heard this sound , the landing party discovers strange creatures . Looking like little more than loathsome blobs of jelly , they emit an unwholesome buzzing , and employ a crude , wingless flight . A phaser at force 3 , on the kill setting – sufficient to destroy most organisms – barely affects these creatures , even after several seconds of exposure . And the creatures do not even register on Spock ' s tricorder . Thinking it may be a trap , Kirk orders the landing party out of the infested area ; as they leave , a creature strikes Spock in the back , leaving a strange puncture wound . Kirk rips it off of Spock ' s back and asks the Vulcan if he is all right . Spock looks upward in pain . Act Two . McCoy removes a small strand of tissue , and then , over Nurse Chapel ' s objections , he closes the wound . The creatures attack by stinging ; they leave behind a piece of this tissue that rapidly infiltrates the victim ' s entire nervous system , far too completely for conventional surgery to remove it . McCoy explains this to Kirk on the bridge while showing him the piece of tissue in a container . Spock recovers consciousness , rages out of sickbay and storms the bridge . His goal : to take the ship out of orbit . Spock is ultimately overcome , sedated by McCoy after being held down by Scott , Sulu , and Kirk , and he is returned to sickbay , where McCoy makes another grim discovery . The K3 indicator , a measure of pain , is very , very high . The reason for the madness is confirmed : victims are in such agony that their minds eventually break under the stress . Spock , recovering consciousness , now claims the ability to control the pain . But after his visit to the bridge , Kirk isn ' t sure and has him continued to be confined to sickbay . Spock , conquering the pain , breaks out of sickbay and plans to visit the planet ' s surface . Scotty , acting on Kirk ' s orders , refuses to transport him . A scuffle breaks out , including Spock using the Vulcan nerve pinch on Leslie . Scott holds Spock by threatening to stun him with a phaser and calls for Kirk to come down . Later , when Kirk appears , Spock explains that his plan is to retrieve a creature for study . He believes that since his nervous system is already infiltrated , there is little more the creatures can do to him . Kirk is convinced , and over McCoy ' s objections , Spock beams down to collect a creature for study . Act Three . Spock is attacked by a colonist on Deneva almost immediately after being beamed down . The man knocks Spock ' s phaser out of his hand with his club . Spock is able to incapacitate the crazed man with a Vulcan nerve pinch and , upon regaining his phaser , begins to struggle against the pain of the parasite . Spock returns with a creature he fires his phaser at and begins to study it . Immediately , in one of the " Enterprise " s science lab s , he realizes that the creature resembles , more than anything , an enormous brain cell . Kirk catches on immediately : these creatures are not " separate " animals , they are " all parts of a single entity " , connected together in some mysterious fashion . This is how it resists phaser fire : each part draws strength from the " whole " . McCoy ' s multiple efforts to find some method to kill the creatures fail . Not heat , not radiation – nothing can kill it . Kirk knows that if they cannot find a way to kill these creatures , he will be forced to destroy Deneva to prevent their spread . A million people will die if nothing can be done . Kirk cannot let the creatures spread , and has absolutely no wish to kill the Denevans , including his nephew . He demands a third alternative . Act Four . The key lies in exploring the properties of the sun . The Denevan the " Enterprise " contacted earlier was free of the creature moments before he died ; something in the sun killed it . It is not radiation , it is not heat – could it be light ? After turning a computer in his quarters on and off , Kirk thinks that it may be the answer . McCoy rigs a test cubicle , puts the sample creature inside , and confirms the theory : high intensity light is fatal to these creatures . Spock enters next ; it is necessary to see what will happen to tissue that has infiltrated a victim . Spock volunteers to enter the cubicle – McCoy , who would prefer his guinea pig be someone other than the man he regards as Starfleet ' s best first officer , attempts to dissuade him , but Spock insists . This test does , in fact , succeed : the blinding light completely frees Spock of the creature and the pain – but at the cost of his eyesight . Spock , exiting the cubicle , remarks that it is an equitable trade – the closest he comes to revealing how much pain he has been experiencing . Only then is the true tragedy revealed : lab tests indicate that the creatures are vulnerable only to a specific subset of the light spectrum : ultraviolet light is its Achilles ' heel . McCoy is chagrined to realize that Spock need not have been blinded at all . Despite this , the answer is at hand . Kirk orders satellite control to deploy a formation of 210 ultraviolet satellite s at 72 miles altitude , in a permanent orbit . The satellites are turned on ; the creatures begin to fail , to fall , to smoke and to die . Ground stations on Deneva quickly make contact ; the creatures are dying everywhere . Spock returns to the bridge – he can once again see . It seems that an inner eyelid , an hereditary trait of Vulcan s , protected his eyes automatically . Relieved beyond words , McCoy asks Kirk not to mention his previous " best first officer " statement – at which point Spock turns around from his science station and thanks the doctor for his compliment , and Kirk jokes that all the concern over Spock ' s eyes had led McCoy to forget about his Vulcan ears . Kirk has Sulu take the " Enterprise " to warp factor 1 and the ship leaves Deneva . Memorable quotes . " " Pain is a thing of the mind . The mind can be controlled . " " " " Freeze right there , Mister Spock . Or I ' ll put you to sleep for sure . " " " " Jim , that man is sick ! And don ' t give me any damnable logic about him being the only man for the job . " " " I " don ' t have to , Bones . We both know he is . " " " " I cannot let it spread beyond this colony even if it means destroying a " million " people down there . " " " " I am free of it and the pain . And I ' m also quite blind . An equitable trade , doctor . Thank you . " " " " My first sight was the face of Doctor McCoy bending over me . " " " " Tis a pity your brief blindness did not increase your appreciation for beauty , Mister Spock . " " " " You ' ve been so concerned about his Vulcan eyes , doctor , you forgot about his Vulcan ears . " " Links and references . References . 2067 ; 2150s ; 2265 ; 2266 ; advisor ; appendix ; archaeologist ; asteroid belt ; autonomic system ; bee ; Beta Portolan ; Beta Portolan system ; bio - bed ; biolab ; blindness ; " Bones " ; brain cell ; call sign ; candlepower ; ; club ; delirium ; Deneva ; Deneva colony ; Denevan ; Denevan ship ; Denevan sun ( aka ) ; dog ; electromagnetic spectrum ; engineering advisor ; engineering control ; environmental control ; Federation ; force 3 ; goggles ; gravimetric pull ; GSK 783 ; heat ; inch ; Ingraham B ; Ingraham B vessel ; inner eyelid ; interception course ; K3 indicator ; Levinius V ; life science ; logic ; mask ; mass insanity ; medical ; mile ; Milky Way Galaxy ; miner ; neural parasite ; nervous system ; optical nerve ; pain ; physical law ; plate ; population ; private transmitter ; puncture ; radiation ; research biologist ; satellite control ; science department ; science lab ; security restraint ; sedative ; shadow ; single - celled organism ; spinal cord ; Starbase 10 ; ; subspace frequency ; subspace transmitter ; surgery ; tentacle ; test cubicle ; Theta Cygni XII ; tractor beam ; tranquilizer ; transmitting station ; trevium ; tricorder ; tri - magnesite ; ultraviolet light ; ultraviolet satellite ; ventilator ; Vulcan s ; ( planet ) ; ( star ) ; Vulcan nerve pinch ; wasp ; white light