The goal of One is for the player to discover the identity of the main character John Cain. The only clue available is a barcode on the player-character's neck. At the beginning of the game, John Cain awakens on the floor of an apartment building, with a gun in place of one arm, no memory, and police helicopters shooting missiles through a window. The player is pursued across the city and country-side by military and police forces who are apparently intending to kill Cain. To combat the police and military forces and other enemies, the player has the option to use Cain's newly installed arm cannon, or rely on more traditional fighting weapons like punch and kicking combos.
Source: Wikipedia, "One (video game)", available under the CC-BY-SA License.