Mark (Doctor Who)
Mark was a participant in a plan to colonize another world. His real name was John Crichton. Prior to joining the program, he had been an athlete. He was one of the three people aboard what the group thought was a spaceship three months on its way to a new, Earth-like planet, where he hoped to begin a new, pollution-free life.
When Sarah Jane Smith ended up aboard, Mark was the first person she met, although she didn't recognize him as Crichton until after he introduced her to Ruth and Adam. When Sarah claimed to have been brought aboard the ship against her will, no one believed her and she was locked in the Reminder Room. Despite this, she became friends with Mark, and was eventually able to prove to him that the "ship" was a fake, built underground. He covered for her when she escaped.
When she was brought back by General Finch, it seemed to Ruth and Adam that she'd never left. Mark's efforts to back up her story resulted in Ruth ordering him placed in the Reminder Room as well, fearing that Sarah's "dangerous influence" was spreading. A concerned and suspicious Adam however contacted Charles Grover on what he thought to be a neighboring ship. Grover pretended to come over wearing a spacesuit, and spoke to Mark and Sarah alone in the Reminder Room, unaware that Adam was listening. When he explained Operation Golden Age to them, Mark was horrified at the thought of erasing billions of innocent people, and refused to cooperate.
The two were left in the room by Grover, but freed by Adam who'd overheard what Grover said. The three were eventually able to prove to Ruth and the others that the ship was a sham and that instead of journeying to a new world, they were still on Earth underneath London, and that Grover was preparing to use Professor Whitaker's Timescoop to turn back time outside of London. Mark joined Ruth and Adam in accompanying Sarah to storm Whitaker's laboratory so they could prevent this from happening.
At the same time, the Doctor also entered with UNIT and Brigadier Lethbridge-Stewart. Refusing to give up, Grover activated the Timescoop, but accidentally sent himself and Whitaker back in time instead of the entire planet. Presumably, Mark and the other colonists returned to their normal lives and their real names.
