A band of cyborgs is terrorizing the planet. Believing that Magneto is controlling the cyborgs, the X-Men try to defeat him. The player can take control of Wolverine, Storm, Cyclops, Iceman, and Gambit as they prepare for the battle to save the planet.
Characters can be changed at any time during the game, which is important as their power level decreases through the game. When the player changes character, those are not being controlled wil gradually recover their power level. Also, some areas can only be completed by using the unique abilities of certain characters.
An alternate game mode has the player facing all the boss characters from the main game, and if the current character runs out of power, they cannot be used again.
Source: Wikipedia, "X-Men: Mutant Wars", available under the CC-BY-SA License.