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.

Episode comprises:
The Sontaran Stratagem: Martha Jones summons the Doctor back to modern-day Earth, but an old enemy lies in wait.

The Poison Sky: As the Sontarans choke the Earth, the Doctor battles to keep both Martha and Donna alive.

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.

The Unicorn And The Wasp: In 1926, Agatha Christie disappeared for 10 days. Was it amnesia? A nervous breakdown? Or a Giant Alien Wasp..?

Old friend Martha summons The Doctor to Earth following her adventures with Torchwood

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