Aaron Crossland was a tech entrepreneur who established the think-tank known as the Key. The Key eventually closed down, leaving Crossland desperate. He was contacted by the Reach, an alien creature who broke through a fissure in time and space into London. The Reach took control of Crossland, and created a trap for the Tenth Doctor, to use his emotions and experiences to create a feedback wave that would wipe out the Reach's species.
Their plan was almost successful, but the Tenth Doctor, along with the help of Gabby Gonzalez and the Reach's species, interfered. As the Reach was about to kill the Doctor and Gabby, the Doctor told Crossland that half of London would be destroyed in the feedback wave. He convinced Crossland to regain control of himself, and Crossland threw himself at the Reach, severing the dimensional link and causing an explosion that killed them both. (COMIC: Revolving Doors)