Inhabiting a colony spaceship in the thirty second century are members of a religious cult which left Earth to find a world of their own. Their leader, Seraph, has downloaded his mind into the ship's computers, but now he has gone silent, enticed and serenaded by a siren song coming from inside a black hole. Trapped in orbit around the void, Seraph's followers are confused by his silence, and when the Doctor arrives, he finds a world on the brink of chaos.