In Gabriel Chase, a mansion in Victorian England, the severe Mrs. Pritchard descends into the basement. She opens an automated door and serves dinner to a silent creature confined in a cell. Above, Ms. Grose, the housekeeper, welcomes Reverend Matthews to the house. The Doctor and Ace land in a toy room.
Mrs. Grose hurries out, unwilling to stay after dark. When the clock strikes six, maids emerge and Josiah Smith, the owner, and Gwendoline, his ward, stir in the attic.
The Doctor and Ace wander the halls and find a snuff box on the floor that is radioactive. They are suddenly interrupted by an explorer. He claims to be on the trail of Redvers Fenn-Cooper, trying to save him from Josiah Smith.
Reverend Matthews angrily confronts Pritchard about being kept waiting but Gwendoline placates him. In the cellar, Nimrod, a neanderthal, checks on the prisoner. It suddenly speaks.
The explorer takes the Doctor to an office and pulls a gun on him. He tells he Doctor of going to Africa and seeing a bright light that burnt into his mind. But before he can shoot the Doctor, he sees his reflection in the window and realizes he has found Redvers. Mrs. Pritchard arrives and takes Redvers away. Nimrod apologizes for the problems and leads them to the drawing room.
There, Matthews mistakes the Doctor for Smith and begins raving about Smith's Darwinian theories. The Doctor has Gwendoline take Ace to change into proper clothing and gives Nimrod a bear fang to impress him. Smith arrives and dims the lights, his eyes concealed by glasses.
Redvers is tied up in a straight jacket. The snuff box opens and a bright light streams into the room. His screams of terror draw Ace, Gwendoline and the Doctor. Pritchard tries to keep them out but Nimrod opens the door. He and Pritchard, however, keep the Doctor from interfering.
Nimrod returns to the cellar where he pulls back the curtains to reveal a glowing orb embedded in the wall. He bows before it but is attacked by something emerging from the shadows.
The Doctor and the others sit down to dinner but are interrupted by the escaped creature calling Smith from the cellar.
Ace calls the Doctor into the hallway. She has realized they are in Perivale and is furious that he has brought her to a haunted house she visited a century later. She sensed an aura of evil in the house. But the Doctor helps he realize she also sensed something alien.
Smith calls Matthews to the attic and tells him that he will have to wait to discuss Smith's theories. Pritchard chloroforms Matthews.
Ace tells the Doctor she visited the house after her best friend had been burned in a racial attack. But before she can say more, Josiah arrives and she runs, eventually taking the elevator into the cellar.
Josiah, whose skin is sloughing off, tells the Doctor he has an enemy in the cellar. He offers the Doctor money to destroy it.
Ace finds the stone chamber and unconscious Nimrod. Two hideous creatures in tuxedos emerge from the shadows and advance on Ace, urged on by the voice of Smith's enemy…