The "Ultimate" answer to every what if any Marvel fan has ever asked...
The game starts out fast. The S.H.I.E.L.D. Helicarrier is under assault and Nick Fury (at this point, still head of the organization since the events of Civil War were a ways off from happening) sends Captain America, Thor, Spider-Man & Wolverine to handle things. Appears that the ole Doctor Doom is at it again, only nobody really gets what he's up to other than it's no good. (Trust and believe when you finally get what he's up to, it will be a MAJOR shock.) Doctor Doom has gotten some of the Marvel world's most nefarious villains from Thor's ne'er do well cousin Loki to Fing Fang Foom to Mysterio to join in on his effort for world domination. Your job, as simple as it is, decipher what Doom's after and stop it. (Oh, if only it were that simple.)
The beauty of this game is that as straightforward as it appears, it's far from that. The deeper you get into the game, the more you realize, the plot's much greater than you can begin to imagine. You're going to go all over, from Latveria to the Orient...Orient to Atlantis. Yes, you'll be saying hello to those Atlanteans and Namor. When the heroes finally run into Doom face to face...well, let's just say it's not pretty and your battle with him will be the farthest thing from it. (I cannot in good conscience spoil it, the turns this story takes and where it takes you will get you.) It goes without saying that you're going to run into a lot of familiar faces, including the Shi'aar Empire and two of their illumiaries, Gladiator and Deathbird.
All in all, this game plays just like X-Men Legends and XML 2. You'll get bonuses for pairing four individuals together, such as four X-Men and Avengers. Anything beyond that, you're going to have to do some researching as even I didn't get all the possible pairings that are involved. You can also create your own, although when I beat it, I didn't really delve too deep into that feature. What I can tell you is that after you end the game, The Watcher will go back through and show you exactly what you did and what impact it had on the Marvel Universe. (So take note, there are certain times in the game where not doing something will prove costly down the line. The Watcher will give you the where and how.) I will tell you one thing...helping out Namor will help ease tension between his people and the 'surface dwellers'. That's all I'm going to spoil.
The boss battles are challenging and the deeper you get into the game, the tougher they will get. (Not for nothing, you will get help with one battle in particular and it's not a question of 'will I need it'. When you see who you're facing, you'll be glad they're assisting.) The mash-ups your imagination can come up with are great, all in all, this is a great game. The end will leave you scratching your head to some degree, especially considering the direction they chose to go in for the sequel, but it isn't enough to have you forget that this was and is an exceptional game. If you can find it, cop it. Expeditiously.