The Doctor tells Steven a story about himself and his granddaughter Susan.
The Doctor and Susan have just been to the theatre and are lost in London. There is a heavy fog, and they cannot hear any signs of civilisation. When they double back, they are still lost.
Suddenly they hear singing, and try to find the voice. They find themselves in an avenue, but their relief turns to dread when they see the buildings' windows are smashed and there is debris in the street. The Doctor briefly sees his and Susan's reflections in a piece of broken window, and is startled to see a wicked and cunning face in place of Susan's. He cries out in alarm. They hear the singing behind them, and turn to see a young lady. She offers to take them home for tea, and introduces herself as Joan.
Joan's house is dilapidated and cold. Her mother is in the living room, ironing. She seems sad, and doesn't seem to want the Doctor and Susan there. He offers to leave, but she sends Joan to make tea. Joan pours five cups, and suddenly the Doctor and Susan realise that there is an old man, Joan's Grandpa, in the room. Joan's mother tells them he was in the last war. She starts to talk about World War II as if it were still occurring, and the Doctor begins to wonder if he and Susan have somehow time travelled.
Joan's mother acts strangely, as if she is warning Joan of something. The Doctor looks at Grandpa, and he looks like the reflection the Doctor had seen. He realises that whatever is going on here, the old man is the source. The Doctor wants to leave, but Joan's mother won't let him. He asks the year, and she says it is 1943. However, the Doctor says it can't be 1943, because the Calders have plenty of sugar, which was rationed in 1943.
Flames start in the room, and Joan and her mother are burned up. The house disintegrates, and the Doctor and Susan rush for the door. Grandpa tries to stop them, and the Doctor yells for Susan to get the mirror. She breaks it, and Mr Calder collapses and scatters into ash.
The Doctor tells Steven that he thinks something in the mirror found Mr Calder. He likes to think that he and Susan freed Joan and her mother from "a nightmarish existence".
- Mr Calder is believed by the Doctor to have done something terrible during World War I.
- Interestingly, the first story in the Short Trips: A Universe of Terrors anthology also featured a mysterious and (yet) unidentified entity trapped in a mirror.