A nameless, nebulous life form existed in "every reflective surface" of the artificial sea on Mars. It had no name and was not alive to give names to, but was on the verge of being something real. Sometimes the life form was many individuals, sometimes it was one, and sometimes it didn't exist at all. It could take over human and Time Lord bodies, but after an hour of taking a body, it broke apart into the same liquid the red-coloured sea was made from, and went back to the nothingness to wait. The reflections were dependent on a stable image refracted through the air.
In the 22nd century, Susannah Chambers' father found the life-replicating life form on the Martian shipping lanes. Within the hour, before his body broke down, the sea used the body of Susannah's father to lure the private luxury ship of Alvar Chambers towards it. Alvar's ex-wives believed that this "something" could keep Alvar alive. The life form took over the bodies of Susannah and Vikki, and later Alvar, and "Alvar" ordered the Ninth Doctor to be thrown into the sea, where it also took on the Doctor's body. While controlling the Doctor, it said that Mars' air being "poisoned" by humanity and its sea being coloured was what let the "something" out, and as "Alvar" and "the Doctor", the being killed Alvar's doctors, Latimer and Godwin. It later broke out of mirrors to take over the bodies of Alvar's seventh wife and the Doctor's companion Rose Tyler, and made Alvar's second wife crawl inside his body to prolong the body of Alvar.
The life form "seduced" its victims, and played a reflection of their lives, what might have been, and what they feared might have been, and ate the victim alive while they were distracted by this dream. In Rose's example, it involved her repeatedly turning down the Doctor's invitation to travel with him until she became an old woman. It learnt about the Doctor and the TARDIS and planned to "gorge" on everything that lived throughout time and space. Rose escaped, and created a high-frequency blast with the sonic screwdriver to interfere with the image, forcing the life form into a single body. She went back into the life form and freed the second and seventh wife and Vikki, with Alvar and Susannah being unable to make it, while the Doctor created a tornado using the ship's artifical air settings, making the air bubble surrounding the ship, which the life form used to refract light off of, unstable. The Doctor didn't believe it was dead, and that it was still out there, but hoped that this meant that the human race might start treating Mars with respect now. (COMIC: The Cruel Sea)