Terror of the Zygons ( TV story ) Terror of the Zygons was the first story of season 13 of " Doctor Who " ( although it had been originally produced to conclude season 12 ) . The story saw the departure of Harry Sullivan as a companion and was the first UNIT story since " Robot " . Although two more UNIT stories followed later this season , this was the last appearance of the Brigadier until 1983 ' s " Mawdryn Undead " . Synopsis . When the Doctor , Sarah Jane and Harry arrive in Scotland , having received an urgent request for assistance from the Brigadier , they discover that the mysterious force which has destroyed three oil rigs has left giant teeth marks on the wreckage . The mystery deepens , leading them to the shores of Loch Ness where they find that the legendary monster really does exist — and is the murderous tool of the Zygon s , aliens intent on overpowering the planet . The Doctor , his companions and UNIT must find a way to defeat the deadly Loch Ness Monster and its controllers , but the Zygons have the terrifying power to change shape . The Doctor ' s life has never been in more danger , as the line between allies and enemies is tested to the very limit . . . Plot . Part one . On an oil rig off the coast of Scotland , a strange , high - pitched beeping sound echoes through the structure . It begins to break apart , collapsing into the North Sea . The Fourth Doctor , Harry and Sarah hitch a ride with a motorist to the town where the Brigadier and UNIT have set themselves up in an inn . The Brigadier is talking to Huckle , an official of Hibernian Oil , who owned the rig . Huckle complains that three rigs have been destroyed in the last month , two of them belonging to Hibernian . The Brigadier assures Huckle that UNIT is also concerned , especially about the loss of life . The Doctor , Harry and Sarah arrive at the inn . It turns out that the man who has given them a lift is the Duke of Forgill , a local landowner . The Duke is angry about Huckle ' s men trespassing on his land . Huckle assures him that he has warned his employees about this , and any who are caught will be dismissed without question . However , the Duke warns that any of Huckle ' s men caught on his land will be shot . He also wants to know what UNIT are doing here , but the Brigadier says that it is a sensitive matter . Though not pleased , the Duke leaves . On the shoreline , a survivor of the destroyed rig washes up , barely alive . The Brigadier brings the Doctor , Harry and Sarah to Hibernian Oil , where Huckle briefs them on the injuries of the rigs ' crew . Harry decides to examine the injuries at the sickbay , while Sarah goes to talk to some of the locals . Huckle cannot understand it : the rigs were designed to be unsinkable and the seas were calm and empty before the incidents . The Doctor darkly notes that the sea is never empty . Back at the inn , Sarah talks to Angus , the landlord . She admires a stuffed stag ' s head above the fireplace , a gift from the Duke just this past week . Angus observes that the Duke has not been himself since the oil companies came . His servants have all left to go and work for them , leaving Forgill Castle cold and empty . He tells Sarah about Tulloch Moor and people disappearing over the centuries when the mist comes down . Sarah is sceptical — evil spirits do not destroy oil rigs . All this while , something alien watches Angus and her on a monitor screen . The Duke ' s man , Caber , sees the rig survivor , Munro , stagger from the surf . Harry is driving by and goes to Munro as he collapses . Before Munro can tell Harry about what smashed the rig , Caber fires a rifle , killing Munro and creasing Harry ' s forehead with a second shot . Harry falls , unconscious . Back at the inn , the Doctor is working on a radio probe to check for localised jamming when a call comes in informing them about Harry . Alien hands manipulate strange , organic controls . A high - pitched beeping summons a creature from the depths . It heads towards the Ben Nevis rig . The rig ' s communications with Huckle are jammed . The Doctor and Sarah visit Harry , who is sedated and being tended by Sister Lamont , the nurse . The Brigadier arrives to tell him of the Ben Nevis rig ' s destruction . Sarah stays with Harry while the Doctor goes with the Brigadier . Outside , the Doctor spots wreckage from the rig with odd holes in it . He asks Sergeant Benton for plaster of Paris and makes a mould of the holes , revealing that something with very large teeth bit the rig . The watching aliens decide that the Doctor knows too much and must be destroyed . Harry awakens . Sarah goes to tell the Doctor while Sister Lamont tells Harry he will be very well looked after . A high - pitched beeping echoes through the room , and Harry looks up in fear . As Sarah calls the Doctor , a suction - tipped hand clamps down on her shoulder . She turns as she is grabbed by a large , orange biped creature . Part two . The Doctor hears Sarah ' s cries over the telephone and rushes to the sickbay . Sister Lamont tells them that she found Sarah gone and Harry ' s bed empty . The Doctor snoops around and finds Sarah unconscious in a decompression chamber . As Sarah comes round and starts telling the Doctor about what happened , a Zygon locks them in and starts to remove the air from the chamber . To escape death , the Doctor hypnotises Sarah into not needing to breathe and then puts himself in a similar trance . Harry is brought to the Zygon ship deep underwater , where he meets their warlord , Broton . Centuries ago , their spaceship was damaged , and they landed on Earth . They were waiting for rescue when they heard that their world had been destroyed in a stellar explosion . Now , they intend to claim Earth for their own , using the sea monster under their command , an armoured cyborg of great power . Broton explains that the Zygons depend on the lactic fluid of the Skarasen . Harry realises that if the monster is destroyed , the Zygons will die . Broton declares that no human weapon can affect the Skarasen . The Brigadier is briefing one of his officers at the inn when gas floods the room , knocking them all out . Meanwhile , Benton has found the Doctor and Sarah . He opens the pressure door . The Doctor exhales , awakens and carefully revives Sarah . He tells Benton the trance was a trick he picked up from a Tibet an monk . The Doctor , Sarah and Benton find the entire village has been drugged by some kind of nerve gas , but they soon revive . The Doctor deduce s that this was done so that something could get around unseen . The Zygons watch Huckle give the Doctor a device he found in the wreckage : the signal device that summons the Skarasen . Broton says it must be recovered . Harry is taken to another room in the ship , where he sees other humans hooked up to booths , providing the Zygons with " body prints " that they use to assume human form . The Doctor hypothesises that the signal device sends out a primeval mating call to attract the beast . The savaged body of a UNIT soldier is found on the moor , and the Doctor and the Brigadier go to see it , leaving Sarah behind in case Harry turns up . He does , but it is his Zygon duplicate Madra . He takes the signal device , saying that the Doctor asked him to collect it . Sarah notices his curious behaviour and questions him , but he pushes her aside and runs away . Sarah and some soldiers pursue him . Sarah discovers " Harry " hiding in an upper level of a barn , and he attacks her with a pitchfork . She steps aside at the last moment , and he falls , impaling himself on his weapon and turning back into a Zygon . The signal broken , Broton realises Madra is dead and remotely disperses the corpse . Harry , however , is freed from his body print booth . Sarah wonders how the aliens knew they had the signal device . The Brigadier orders the inn searched for bugs . Broton in turn orders the Skarasen unleashed , even if it reveals their presence . The signal device begins to beep . The Doctor grabs it to lure the creature away , while the Brigadier tries to get a fix on the activating signal . The Skarasen catches up to the Doctor on the moor , and he finds that he cannot get rid of the semi - organic signal device which has fastened itself to his palm . The Brigadier traces the incoming signal to Loch Ness . As the Doctor stumbles , Broton orders the Skarasen to destroy him . Part three . Harry rushes into the Zygon control room and randomly hits controls , making the device fall off the Doctor ' s palm and allowing him to roll out of the Skarasen ' s way . Since the device is dead , Broton assumes that the Doctor is too and recalls the Skarasen . The Doctor retrieves the device and makes his way back across the moor as the Skarasen glides away . In the ship , Harry is dragged away by the other Zygons . The Doctor is met halfway by the Brigadier and Sarah , who tell him that the signal came from Loch Ness . The Doctor decides to visit Forgill Castle , right next to the loch . They get a frosty reception from the Duke , who does not believe them about the monster , aliens and their intention to drop depth charges in Loch Ness . At the inn , Angus discovers the Zygon surveillance link in the stuffed deer head that was the Duke ' s gift . As he tries to pry it loose , Sister Lamont enters the inn . The sister turns into a Zygon and kills him , removing the link after . Angus ' dying cries are heard by Benton and some UNIT soldiers , who discover the body and pursue the killer . The UNIT troops sweep through the nearby forest and fire on the Zygon . The Brigadier is told a Zygon is being cornered . He and the Doctor leave Sarah at the castle to research the monster . In the woods , the wounded Zygon disguises itself again as Sister Lamont and fools a UNIT soldier long enough to knock him out and steal his jeep . At the inn , the Doctor notices the missing eye in the deer head and realises that the Duke may be a Zygon facsimile . At the castle , Sarah examines the upper bookshelves . She triggers a hidden switch , and a section of the bookcases slides open , revealing a passage . She takes a torchlight and goes in , following a long dark tunnel that leads to the Zygon ship . " The Duke " ( in reality Broton ) finds the open bookcase . He and " Caber " take the wounded " Sister Lamont " ( the Zygon Odda ) into the ship ; the crew must be alerted and the intruder found and destroyed . Sarah finds the cell where Harry is held . She frees him , and they sneak back to the castle , just in time to meet the Brigadier and the Doctor . Sarah tells the Brigadier about the Zygon ship . The Doctor enters the tunnel but does not get far ; a scream is heard . The Zygons emerge in their true forms . Broton tells the humans that they are leaving with the Doctor as their prisoner . He warns them the oil rigs were only a test ; the " big event " is yet to come . Broton retreats , sealing the entrance to the tunnel . The Brigadier orders the loch depth - charged to bring the Zygon ship to the surface . The Zygon ship does rise but keeps on rising into the sky , flying away . Part four . Broton orders a jamming signal transmitted to block the humans ' radar systems . The Brigadier prepares to move his troops out , but Sarah and Harry suggest they search Forgill Castle before they leave for clues to the Zygons ' plans . Sarah finds some papers that indicate the Duke is the President of the Scottish Energy Commission , but Harry dismisses the information as useless . They return with the Brigadier to London . The Zygon ship lands in a disused quarry . UNIT cannot track it . Radar stations are knocked out all over the country . However , there are reports of a large underwater object heading south at high speed . Broton enters the Doctor ' s cell and , taking the Duke ' s form again , tells him that Phase I of the conquest of Earth is complete . A great Zygon refugee ship is on its way but will take some centuries to arrive . In the meantime , Earth ' s environment must be restructured with human labour and Zygon technology to turn it into a new homeworld . Left alone , the Doctor rigs some of the organic technology in his cell , electrocuting himself but sending a transmission to help UNIT track the ship to the quarry . By the time Broton and his men open the Doctor ' s cell door , the Doctor is apparently dead . However , when Broton leaves , the Doctor comes to . He makes his way to the body print chamber and frees the real Duke , Sister Lamont and Caber . Broton leaves the ship to plant a trilanic activator on his target , which the Skarasen will then destroy , proving the power the Zygons hold . When Phase II is complete , he will broadcast his demands to the world . Inside the ship , the Doctor uses his sonic screwdriver to set off the fire sensor , triggering an alarm . When the Zygons go to investigate , the Doctor ushers the humans out of the ship , jamming the hatches and activating the ship ' s self - destruct . The Brigadier has arrived , and the escapees reach UNIT ' s position . The Doctor shouts for everyone to get down , just before the Zygon ship explodes . However , Broton is still free and has control of the Skarasen . The target is in London and must be close to the River Thames . The Brigadier says the Prime Minister will be at a conference at Stanbridge House , near the river . The Duke adds it is the first International Energy Conference , with delegates from all over the world . As the Duke is the President of the Scottish Energy Commission , Broton will have a pass into the meeting . They leave for London . " The Duke " places the activator in the basement of Stanbridge House . The Skarasen has been sighted mere minutes away . The Doctor finds Broton , back in his true form , who swears he will make sure the Doctor is dead this time and attacks him . Sarah calls for the Brigadier , who arrives to shoot Broton dead . The Doctor finds the trilanic activator in his pocket and feeds it to the Skarasen as it rises out of the Thames . After it consumes the device , the beast sinks back into the river and heads back to Loch Ness , the only home it has ever known . However , it is witnessed by not only the Doctor , Sarah and Harry , but many terrified Londoners . Returning to Scotland , the Brigadier tells the Duke that the incident will be kept quiet , even the appearance of a fifty foot monster in the Thames . The Doctor leads them into the woods to the TARDIS and offers all of them a lift . The Brigadier and Harry decline . Sarah agrees on the condition they go straight back to London . The Doctor promises , and the TARDIS takes off . The Duke tells the Brigadier that , as a Scotsman , he should have gotten a refund for Sarah and the Doctor ' s return train tickets . DVD , Video , CD and Laserdisc releases . DVD releases . A DVD was commissioned in April 2011 , and subsequently announced for release in the UK on 30 September 2013 . However , an " advance release " , minus extra features , was also announced for release on 29 July 2013 as part of the UK - exclusive " The Fourth Doctor Time Capsule " box set , limited to 5 , 000 copies . At the time it held the distinction of being the final complete classic - era story to see a DVD release , before " The Enemy of the World " was announced as having been found . VHS releases . This story was released as " Doctor Who : Terror of the Zygons " on VHS . Released : Notes : Laserdisc release . Released :