Harry Sullivan and Naomi Cross are on Earth at two different points in a timeline that has gone badly wrong. Back in his own time with UNIT, Harry is in the Lake District, unaware of the horrors to come. Four decades on, Naomi is adrift on an ocean in an apocalyptic nightmare.

What is causing the Earth's unnatural end? As the Doctor scours the years, he meets someone from his past with the tools to help - a mechanic called Ray.