A meeting in a London bus with jewel thief Lady Christina takes a turn for the worst for the Doctor when the bus takes a detour to a desert-like planet, where the deadly Swarm awaits.
In a Mars base the inhabitants are being infected by a mysterious water creature which takes over its victims. The Doctor is thrust into the middle of this catastrophe knowing a larger one is waiting around the corner.
The Ood have given a warning to The Doctor. The Master is returning yet that is not the biggest threat. A darkness is coming which brings with it The End of Time.