At the outset, Mercer is a much easier character to control than he is to understand, especially if you opt to play with an Xbox 360 controller rather than a mouse and keyboard. Making giant leaps, gliding through the air, and even running up the sides of skyscrapers are effortless actions. You could probably make it from one end of Manhattan to the other in a straight line using nothing more than the sprint button if you really wanted to, since it's the only one you need to scale buildings, barge through crowds, and overcome obstacles like cars and rooftop air-conditioning units parkour-style. No fall will ever hurt you, no obstacle is impassable, and there are very few enemies who can keep up with you when you're at full clip. Combat is also relatively simple early on, but as you progress and learn more about who and what Mercer is, his repertoire of moves grows exponentially until remembering which button combinations or keys trigger which moves in which of his five forms becomes something of a challenge. You certainly don't need to remember how to use every single move you unlock with evolution points, which are earned by doing just about anything, but it's unfortunate that even after picking favorites you might find yourself having to hold down up to three buttons (a trigger and two opposing face buttons) simultaneously to perform them. You don't need to do anything quite that crazy on your keyboard, but there are a lot of keys to remember.

Regardless, combat in Prototype is a blast. Using the Function keys or a slick radial menu that slows down the game any time you call it up, you can shape-shift into different forms that morph your arms into blades, a whip, and hammerlike fists. You also have the option to use more conventional weapons dropped by enemies and, eventually, to hijack tanks and helicopter gunships. The latter are especially fun, because you can use a whiplike arm to latch onto them in midair--even while falling from another helicopter that you've been shot down in. One benefit of having all of these options available to you, as well as the ability to disguise yourself as any human character that you "consume" (read: absorb) to regain health, is that many of the already-varied story missions (as well as a couple of the boss fights) can be approached in a number of different ways. For example, if you need to destroy an item inside a military base, you could attempt it in one of the aforementioned vehicles, walk right in there and set about killing or consuming everything that moves, stand on a nearby rooftop and throw things, or even disguise yourself as a soldier and distract the real military by pointing at a nearby character and declaring that he's the enemy. As a soldier look-alike you can even call in airstrikes from your unsuspecting comrades, but you get only a very limited number of these. Some missions, and almost all of the optional challenges scattered across the island, force you to play a certain way, but for the most part you're free to use the environment and everything in it however you see fit.

It's almost impossible to play Prototype without feeling like Mercer is an overpowered character at some point, but any time you start getting too comfortable, there's a good chance the game is about to change things up on you. To give specific examples would be to risk spoiling the game's story for you, but suffice it to say that new enemies and hazards are introduced, moves you've come to rely on might not always be available or effective, and as time passes, Manhattan becomes an increasingly perilous place to be. The difficulty curve is nigh on perfect, and the fact that you're continuously adding new moves to your arsenal to combat new dangers helps keep the gameplay from getting repetitive.