Once and Future: The Martian Invasion of Planetoid 50

The Time War. The Doctor has been injured and brought to a Time Lord field hospital. His body glows with energy, but this is no regeneration into a future form - instead, the Doctor’s past faces begin to appear as he flits haphazardly between incarnations…

Staggering to his TARDIS, the Doctor sets out to solve the mystery of his ‘degeneration’. Who has done this to him? How? And why? From the Earth to the stars, across an array of familiar times and places, he follows clues to retrace his steps, encountering old friends and enemies along the way. Tumbling through his lives, the Doctor must stop his degeneration before he loses himself completely...

The First Doctor arrives in Victorian London amid a Martian invasion. But he discovers all is not what it seems when Missy appears...

Soon, the Doctor is propelled into a future incarnation - the Tenth - but he is not alone in battling Missy and her Martian invaders. The Paternoster Gang - Madame Vastra, Jenny Flint and Strax - are also here to assist. But can they offer the Doctor any help in solving his own degeneration mystery?