Parts: 1 (45-minute)
Writers: Steven Moffat
Airdates: 28 November 2015
Production Code: n/a

Doctor: twelfth

Story: A teleport deposits the Doctor in a mysterious castle. It appears to be completely abandoned — leaving the Doctor with nothing but his own wits to puzzle out who has brought him here, and why — until he discovers the presence of a malevolent entity which follows him, inexorably, with the intention of murdering the Doctor once it can lay hands on him.

With the creature in hot pursuit, the Doctor must evade it while exploring the castle and attempting to solve the mysteries behind his imprisonment.