The story has some alien creatures known as the Phalanx trying to take over the Earth. The X-men have to foil their plot and save the Earth...rather basic, but then most of the plots on the earlier systems are. The Phalanx remind me of the Brood villains the X-men usually fight, but these particular alien beasts use clones to fight. So you are off as soon as you turn the power on in Siberia, a Sentinel factory, Magneto's asteroid, Apocalypse's base of operations, the savage land and finally the alien ship of the Phalanx in your efforts to defeat your enemy.
The game play is a bit more complicated than your basic side scrolling game. I would prefer more linear levels as certain levels are a pain to navigate. Your attack is dependent on which X-men character you choose as Cyclops has a nice range attack as does Gambit. The Beast and Wolverine can really get in close and Nightcrawler can teleport right passed certain obstacles. Psylocke is somewhat useful in a couple of stages as she can destroy some enemies with one hit of her psychic knife. Then later in the game Magneto becomes a playable character and he really can devastate most of the bosses thanks to his ability to simply hover and rain death upon his enemies.
So a fun game, if it were a bit less difficult it would have been better. You die during a stage, you start from the beginning...and let me tell you, some of these stages are quite large. There is a particularly annoying stage where rocks constantly come smashing down on you. Still, this has been the best X-men game I have played on the earlier consoles. It is also better than the Wolverine solo outings on the NES, SNES and Genesis too. Worth a play through if you can locate it.