Hot on the heels of the acclaimed third series, Doctor Who returns for a fourth instalment with a familiar face alongside the Doctor. Award-winning comedienne Catherine Tate returns as the Doctor's new companion, revisiting her role as Donna Noble who featured in the 2006 Christmas special The Runaway Bride. Martha Jones - who left the Doctor at the end of Series Three to break the unrequited circle of love she felt for him - also returns.
4. The Sontaran Stratagem Martha Jones summons the Doctor back to modern-day Earth, but an old enemy lies in wait.
5. The Poison Sky As the Sontarans choke the Earth, the Doctor battles to keep both Martha and Donna alive.
6. The Doctor's Daughter On the planet Messaline, in the middle of an endless war, the Doctor meets the most important woman of his life.
7. The Unicorn and the Wasp In 1926, famed author Agatha Christie disappeared for 10 days. Was it amnesia? A nervous breakdown? Or a giant alien wasp?