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 Doctor returns, alongside newly weds Amy and Rory, to face monsters and mysteries and adventures all across time and space. Together they find themselves in sixties America, battling the invasion the world forgot, then aboard a pirate ship on the high seas of 1696, to solve the mystery of the Siren. In a bubble universe at the edge of reality, the Doctor meets an old friend with a new face.

Unemployed Antonio is elated when he finally finds work hanging posters around war-torn Rome. However on his first day, his bicycle—essential to his work—gets stolen. His job is doomed unless he can find the thief. With the help of his son, Antonio combs the city, becoming desperate for justice.