Parts: 1 (45-minute)
Writers: Sarah Dollard
Airdates: 21 November 2015
Production Code: n/a

Doctor: twelfth
Companion: Clara

Story: A frantic phone call from Rigsy brings the Doctor and Clara to Earth, where they find their friend branded with a tattoo he doesn't remember getting, of a triple-digit number on the back of his neck. Even more sinister than its sudden overnight appearance is the fact that it's impossibly counting down to zero.

The Doctor and Clara are immediately filled with questions — what exactly is the tattoo? who put it on Rigsy, and why? and what will happen when the countdown reaches zero? — and quickly spring into action in an attempt to answer them. Their search will lead them to a secret hidden in the heart of London, but time is running short...