Long ago, a massacre in a suburban house led to the young Harry Sullivan joining UNIT. But the murders were never solved. Years later, Harry and Naomi Cross investigate an oddity in the London Underground and uncover a whole different side to the capital.

Creatures of myth are running amok across the city in a conflict going back decades. And somewhere in the shadows lurks a new incarnation of their old friend... the Doctor. And he's here to stop a war.