The Cathedral of Contemplation is an enigma, existing outside time. It turns through history, opening its doors across the universe to offer solace to those in need. Occasionally, the Doctor drops in – when he's avoiding his destiny, it’s an ideal place to get some perspective. Only this time he's already there from several lives earlier, so when dimension barriers break down, his past and present collide. And when the Daleks invade and commandeer the Cathedral, two Doctors must unite to stop them – or face extermination twice over!