Alex Mercer, also known by his Blackwatch codename: Zeus, he the protagonist of Prototype. He was the former head researcher of the Blacklight Project at Gentek. After learning that those affiliated with the project were being systematically murdered, Mercer attempted to flee the city with a sample of his research contained in a vial. Cornered at Pennsylvania Station, Mercer smashed the vial and unleashed the deadly virus into the surroundings. This triggered a viral outbreak that eventually covered all of Manhattan Island.

After waking up at a Gentek facility with no recollection of his past, Alex Mercer escaped the building, and soon discovered his new powers and abilities, granted by the virus he unleashed. He began to utilize his powers to learn more of his fragmented past, as he scoured all of New York for those responsible.