Donna mysteriously vanished as she prepared to marry her boyfriend in a glittering ceremony on Christmas Eve. She reappeared, much to her astonishment, in the TARDIS with the Doctor.
As the TARDIS raced to the church, it soon became apparent that Donna was the key to an ancient alien plan to destroy the Earth. The episode ended with the Doctor inviting Donna to travel with him ? an invitation which, at the time, was declined?
Now reunited, the Doctor and Donna travel back to Pompeii in AD 79 on the eve of the infamous eruption where people are slowly turning to stone, investigate a series of grisly murders with the help of Agatha Christie, journey to the home world of the sinister Ood and come face to face an old enemy of the Doctor's.
Series Four also sees the return of the Doctor's previous companions ? the vivacious Rose Tyler (Billie Piper) and the feisty Martha Jones (Freema Agyeman).