Doctor Who: Once and Future: Two's Company (4)

Back on Earth, the Doctor finds a clue in the search for the origins of the degeneration weapon. In his Sixth body, he meets Jackie Tyler and Lady Christina de Souza - thrown together by fate, or something more? - in pursuit of a powerful item of jewellery.

Meanwhile, a Time Lord renegade has duped Harry Sullivan into helping him stop the Doctor's travels before they begin - and all are set on a collision course.