The Doctor Who YouTube channel just posted a clip from “Castrovalva,” a four-part serial first broadcast in January 1982. In this clip, the Doctor discovers the recursive geography of the eponymous town when he asks one of its residents to map it.

Previously: Doctor Who Does Trap Streets.

THE DOCTOR: But if the stories are true though, there should be a street in one of these old maps that no longer exists in the real world.

CLARA: Like a trap street, only not.

THE DOCTOR: What did you say?

CLARA: A trap street. You know, when someone’s making a map—a, um, cartographer—uses a fake street throws it into the mix, names it after one of his kids or whatever, then, if the fake street—the trap street—ever shows up on someone else’s map they know their work’s been stolen. Clever, right?

THE DOCTOR: My God! A whole London street just up and disappeared and you lot assume it’s a copyright infringement.

Unlike trap streets, the street exists, and the reason it has disappeared has nothing to do with cartographic copyright. Finding the street takes some doing, as this clip the BBC has made available recently shows; unfortunately, it takes place immediately after the bit I quoted above.