The plot of Prototype centers on a large conspiracy regarding an unnamed mutagenic virus currently infecting New York City. The effects of the virus have been witnessed before in the town of Hope, Idaho in 1969. After being released in Penn Station in New York, the virus spreads through the city, necessitating at first subtle government actions to combat it, then martial law and evacuation. Alex wakes up with amnesia in a morgue under the scalpels of two HAZMAT-suited scientists and escapes dazed into the city. He then begins to come to terms with his powers (including the ability to absorb memories - which is crucial to piecing together his own), and makes contact with his sister, Dana Mercer. Dana assists him in finding members of the conspiracy, which include government officials, military personnel and employees of bio-engineering firm GENTEK, where it is revealed Alex was previously employed.
Alex encounters several characters, not all of which have currently been revealed. His primary opponent is Captain Cross (The Specialist), leader of the Blackwatch special forces branch of the US military. Other characters include Karen Parker, a researcher at GENTEK and Alex's former girlfriend, Dr. McMullen, head of GENTEK, and Dr. Ragland, a pathologist that assists Alex and studies the infected.
As Alex adds more and more nodes to the Web of Intrigue, the virus begins to spread further and further through New York, and the military is forced to step up operations in order to prevent spreading further. The city is evacuated, quarantined and placed under martial law as the military moves in with a full-scale lockdown with tanks, infantry and air support. The virus grows stronger and stronger, mutating citizens of New York into various forms. As the conflict begins to consume the city Alex's actions will play a part in deciding the course of the battle and perhaps the ultimate victor. As the game features non-linear sandbox play, the player is free to explore the city with Alex's superhuman agility through the course of the game.
From wikipedia.