In the 17th Century, a stranded pirate ship is terrorized by a siren-like creature (Lily Cole) who marks people with a black spot on the palm after an injury, puts them in a trance, and disintegrates them with a touch. The Doctor (Matt Smith), Amy (Karen Gillan), and Rory (Arthur Darvill) land the TARDIS on the ship and Rory receives a cut during a disagreement with the pirates. A black spot appears on his hand and the Doctor and Amy prevent him from succumbing to the song of the siren.
Figuring that the siren is using water as a portal, the Doctor tells the crew to hide in the ship’s dry magazine. There they find Toby Avery (Oscar Lloyd), son of Captain Henry Avery (Hugh Bonneville), who stowed away on the ship after his mother died. He has a fever and a black spot on his palm. When they fail to escape in the TARDIS the siren takes control of it. After another shipmate is taken in the dry magazine, the Doctor realizes that the siren is using reflection, not water, to do her thing. So, they get rid of everything shiny, including the treasure.
A storm brews as they set sail. Toby drops a polished crown and is taken by the siren. The others cut themselves to follow. The siren’s touch transports them to an invisible alien spaceship, the crew long dead from disease. They discover a sickbay where the TARDIS sits, and where the crew is in medical care. The siren is the spaceship’s virtual doctor, taking care of the humans after using the black spot as a tissue sample for diagnosis. Amy convinces her to release Rory and uses his nursing knowledge to effect a cure. The captain and his son and revived crew fly the spaceship.
The purpose of the story was rip-roaring adventure and it received mostly excellent reviews. Hugh Bonneville has appeared in many things, perhaps most notably Downton Abbey. Some found it hard to accept him as an evil pirate, and many thought the episode was a comedown after the amazing two-part story kicking off the season. Karen Gillan had a ball swinging on the ship and fighting pirates with swords. Her pirate outfit received favourable notice in reviews, as did the amazing siren (Lily Cole, with her strange and striking looks). The black spot has often appeared as a sign of doom in pirate stories by Robert Louis Stephenson and others. The mermaid of Zennor was a 15th Century story from Cornwall. Henry Avery appeared in the William Hartnell story The Smugglers, though writer Steve Thompson didn’t know that. He discovered the character in a book owned by his son and liked the fact that he had disappeared.