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.
David Tennant
as The Doctor
Lindsay Duncan
as Captain Adelaide Brooke
Peter O'Brien
as Ed Gold
Sharon Duncan-Brewster
as Maggie Cain
Gemma Chan
as Mia Bennett
Aleksandar Mikic
as Yuri Kerenski
Michael Goldsmith
as Roman Groom
Cosima Shaw
as Steffi Ehrlich
Joplin Sibtain
as Tarak Ital
Alan Ruscoe
as Andy Stone
Lily Bevan
as Emily
Max Bollinger
as Mikhail
