Sarah is attacked by a Zygon disguised as Harry.
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 Tibetan 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 deduces 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.