The game is set in New York City as a virus infects people and the military attempts to put an end to it. The protagonist of the story is named Alex Mercer, who has enemy-absorbing andshapeshifting powers. He can take memories, experiences, biomass and physical forms of enemies through absorption. Alex can also shapeshift into more specialized forms for attack, defense or sensory enhancement. All of these together are intended to give players multiple ways to complete their objectives. Parallel to the game's storyline is the ability to play the game as a sandbox-style video game giving the player free roam of Manhattan
Among the enhanced parkour feats Alex can perform are running up the sides of skyscrapers, jumping hundreds of meters, gliding through the air and sprinting at extreme speed indefinitely. Falls do not damage the player no matter the height, but greater ones will visibly deform the ground. Alex may use various weapons, seize and control military vehicles including helicopters, perform various melee attack without shapeshifting, as well as more gymnastic moves such as air combos, sliding along the ground using any human enemy's body and a high-speed rolling cannonball attack. Alex can regain health by consuming biomass, though over time he can regenerate to a limited degree out of combat.
Alex's primary superpower is his ability to shapeshift, transforming parts of his body into a selection of martial implements acquired over the course of the game, either as purchased 'upgrades' or being given them. Offensive powers include the large and powerful Blade, fast razor-sharp Claws (which can also erupt large spikes from the ground), the telescoping Whipfist, Musclemass that augments his strength and Hammerfists. Defensive options consist of a large Shield that needs to regenerate after excessive damage, and a full-body Armor that exchanges agility and speed for sheer toughness; both will allow Alex to plow through most obstacles when active. Visionmodes include Thermal, which allows Alex to see enemies through smoke and other obstacles, and Infected. Any combination of one defensive and offensive power each may be active, but will give away Alex's presence and drop a disguise if any is in use.
The most powerful Superpower is his Devastator ability. This ability requires Alex to be in Critical Mass - either a state of near-death or having excess stored biomass. He also gains a set of Supermoves. These include the Tendril Barrage, which fires impaling tendrils from his body in all directions, the Groundspike Graveyard, which erupts massive spikes from the ground all around Alex and the Critical Pain, which fires a single beam of hardened biomass from his hands to cause damage on a single target.
