The TARDIS is marooned onboard a 17th-century pirate ship whose crew is being attacked by a mysterious and beautiful sea creature. Becalmed and beset by cabin fever, the pirates have numerous superstitious explanations for the Siren's appearance. The Doctor has other ideas but, as his theories are disproved and every plan of escape is thwarted, he must work to win the trust of the implacable Captain Avery and uncover the truth behind the pirates' supernatural fears – and he must work quickly, for some of his friends have already fallen under the Siren's spell...

When a salvage ship attempts to pull the TARDIS aboard when the time vessel's shields are down, an explosion occurs. The Doctor forces the salvage crew to help him find Clara, who is lost somewhere inside his vessel.

The Bank of Karabraxos is the deadliest bank in the cosmos - only a fool or genius would tempt to rob it. Fortunately, for The Doctor, he's both. But nothing even The Doctor has encountered can prepare them for the Teller: a creature of terrifying power that can detect guilt.