The Doctor and Sarah find their way into the Zygon spaceship at the bottom of Loch Ness.
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. The Doctor and he 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 ‘the 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.