November 21st 2059, and Bowie Base One - the first human colony on Mars - is destined for destruction in a nuclear explosion. This tragedy is a fixed point in history. The Laws of Time dictate that it cannot - must never - be changed.

The Doctor arrives just as a viral life-form escapes from the Martian ice into the base's water supply. A single drop can transform a human into a terrifying monster with the power to infect others. History records that the threat is destroyed along with the base and every human in it. But as his darkest hour comes calling, the Doctor resolves to break the rules as he never has before...