Martha Jones summons the Doctor back to modern-day Earth, but an old enemy lies in wait, as Russell T Davies's Bafta Award-winning time-travelling drama continues. With the mysterious ATMOS devices spreading across the world, Donna discovers that even her own family is not safe from the alien threat – but is it too late to save them?

David Tennant plays the Doctor and Catherine Tate plays his new companion, Donna Noble. Freema Agyeman and Christopher Ryan guest star.