On an old Earth colony, Tegan is a captive of Anna in a little room. Anna is the only survivor of her colony, which had been attacked by aliens. She has been alone for a long time. Anna ties Tegan to a chair to keep her from leaving with the Doctor and Nyssa in the TARDIS.
Exploring the corridors, the Doctor and Nyssa find the crashed remains of a spaceship. They think that Tegan and Anna are doing "human things", and are unaware that Tegan is a prisoner.
Anna tells Tegan that the Doctor and Nyssa will leave her behind, just like the other colonists left Anna behind. Tegan tries to convince Anna to let her go. When that doesn't work, she starts to taunt Anna, making her angry. Anna begins to beat on the door, eventually breaking the glass of the window. She huddles in a corner, sobbing, not noticing that Tegan has tipped over her chair and grabbed a piece of broken glass. She cuts her hand badly, but begins to cut at the ropes tied around her wrists.
The Doctor and Nyssa are in the TARDIS; the Doctor is searching for the nearest Earth colony at which they can leave Anna. He is taken aback at what he finds. The nearest Earth colony is very far away.
Tegan has freed herself and runs down the corridor. She feels great relief when she sees the Doctor and Nyssa. They can still hear Anna sobbing a little way down the corridor.
The Doctor goes to the little room while Nyssa tends Tegan's hand. The corridor is dark, which surprises Tegan, because it should be lit. The sobbing has stopped and Tegan wants to see what the Doctor sees. She and Nyssa go to the room, which still has the furniture and broken glass, but there is no sign of Anna.
Back in the TARDIS, the Doctor tries to explain. The attack on Anna's colony happened centuries ago. Sometimes, however, an event can make a dent or reflection on time. His explanations don't make a lot of sense to Tegan, so he finally says that Anna was a dream. They all thought they saw Anna, but she wasn't really there. He notes that whatever psychic event caused this to happen, it affected Tegan most strongly because she is human, as Anna was.
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