The Cuculus lived in purple, egg-shaped spaceships. In the centre chamber was a psychic crystal which they used to implant their minds in other people. These possessed people could be identified by their glowing purple eyes. The Cuculus could drain the minds of people using a purple liquid, which was done by submerging the person in the ooze.
In 1920, the Cuculus invaded Earth, starting with New York City. The Eleventh Doctor was there to stop them, though he wanted the help of Harry Houdini for his escape skills. Using possessed humans, the Cuculus tried to capture the Doctor and Harry, until the Doctor eventually gave up. The Doctor was placed in a case of brain-draining liquid to gain his knowledge of the universe, while Harry was locked in a cell. Harry easily escaped and was able to free the Doctor. The Doctor was able to reverse the psychic crystal, returning all the Cuculus minds into the Cuculus bodies and freeing the humans. They returned the Doctor and Harry to Earth, but also implanted them with a quick spreading virus, hoping to kill the humans that way. However using a crystal Harry had stolen from the ship the Doctor stopped the virus.(PROSE: Houdini and The Space Cuckoos)