The Doctor and Donna parted ways when the Doctor had to wipe Donna's memory, making her forget him, in order to save her life. He left her family with a warning: if ever she remembers, she will die. But with the two coming face to face once more, the big question is, just what brings the Doctor and Donna back together?
Russell T Davies, Showrunner says: “They're back! And it looks impossible - first, we announce a new Doctor, and then an old Doctor, along with the wonderful Donna, what on earth is happening? Maybe this is a missing story. Or a parallel world. Or a dream, or a trick, or a flashback. The only thing I can confirm is that it’s going to be spectacular, as two of our greatest stars reunite for the battle of a lifetime.”