Monsters vs. Aliens follows the plot of the Dreamworks movie it is based on. You're introduced to the monsters (and poor Susan) in the beginning of the game. All those cheesy movies about swamp monsters or mad scientists-turned-bug men - turns out they're all true, and the monsters all live under the secret watch of the U.S. military.
At first, your goal is to escape the secret facility, as the monsters have become bored with their situation. But after a long and drawn-out escape, the General catches up with the monsters. After he recruits you, your goal is now to fight a menacing alien threat. Either way, you'll be doing the same thing you were doing before (you really have to wonder why the alien robots don't differ all that much from the ones the government was sending after you). Susan, the woman turned giant, will skate through levels using cars as the skates. The Missing Link (and yes, that's his only name) will put you through your 3D platforming paces and genrally smash and trash things with his fists and fins. And finally, B.O.B. will use his blob body to stick to walls, walk on ceilings, and do other bizarre things like shoot plasma bullets (not really as cool as it sounds). You alternate between playing these characters, but for each character, the game will pretty much remain the same.
Throughout the game you'll collect monster DNA, which serves as bonus points that you can turn in for bonus features. Movie stills as a bonus might work, but it seems as if they picked the least interesting parts of the movie to take still frames from. You'll also be able to unlock commentary. With this feature, you can replay levels, but with added inner monologue narrative from the character you are playing. That's kind of cool, but you may get tired of playing the same levels over and over just to hear the monologe. One thing that could have been a better motivator is the achievement system. You don't know what achievements you've hit until you finish the level. Even then, it can be confusing as to how you earned them. If they would pop on the screen when you earned one, at least you'd have a better chance at guessing why you achieved something.
There is at least a co-op mode where you can play with a friend. This seems a bit tacked-on, however, as one person will control the regular character for the level and one person just gets a laser where they can shoot things for you. Still, co-op is better than no co-op, and it just might be what saves this game for some people.
