Alien ship (The Curse of the Black Spot)
In a parallel universe, a species created a spacecraft that eventually crashed to Earth and became linked with the Fancy, a sailing ship in the 1890s.
The species who owned the ship were killed off by a disease, which the Eleventh Doctor believed to be a common cold. The ship's interface, which took on the form of a Siren, began transporting crew members of the Fancy over to the other dimension where she could care for them after they'd become injured. Once a person was transferred to the other ship it was impossible to free them, apart from signing a deal with the Siren by being either family or spouse to the person. The dimension could be accessed through "time windows" that appeared all the way over the Fancy. After the Doctor, Amy Pond and Henry Avery travelled to the other dimension, Amy rescued her husband Rory Williams after he'd been taken by the Siren, and Henry remained on the other ship to care for his son, Toby. (TV: The Curse of the Black Spot)
