It's the X-Men, and they're out to stop Magneto and whoever else is bad.
This game stars five of the X-men: Cyclops, Wolverine, Gambit, Beast, and Psylocke. (Professor X is probably coaching them from a safe place, like he always does.) Each member has their own level to complete, but anyone can be chosen to play the remaining levels. The author primarily uses Beast for his power, though Psylocke is chosen in one stage. For more details, see the movie's comments and the discussion forum.