The story is loosely adapted from the X-Men: Onslaught arc: when Professor Xavier was forced to enter Magneto's mind, some of Magneto's anger merged with Xavier's darkest impulses and produced a powerful psychic entity called Onslaught. The game expands on this by having Xavier's own conscience reach out to heroes from the Capcom universe, drawing them into the conflict to help stop the entity Xavier inadvertently helped create.