The game follows Mondo Zappa, a contract executioner who works for the Bryan Execution Firm, in a future world where he receives jobs to kill cyborgs and assassins. The story is told through 12 episodes set in various locations around the world, with animated sequences that further the plot between missions. Completing each main mission opens up several side missions.
The gameplay revolves around Mondo's katana-wielding abilities and his cybernetic arm, Musselback. His right hand is equipped with a katana called Gekkou for attacks and deflecting bullets, while his left arm has a range of operations that require blood absorption from enemies to perform various functions. The game features combos, moon slashes, and puzzle-solving using the cybernetic arm.
After each mission, Mondo receives payment based on his performance statistics. Upgrades can be unlocked through moon crystals, divided into attack skills, sub-weapon skills, and special skills. Players can also equip costumes for various characters in the main office and purchase items from a gift shop with cash earned.
In addition to the main gameplay, there is a mini-game where Mondo takes pictures with Gigolo glasses to seduce women, requiring timing and strategy to complete successfully.
The game features additional content in its retail US edition, including an art book and a soundtrack disc. The PC release includes a Nightmare Edition with the Smooth Operator DLC, as well as a new Nightmare difficulty mode and theater mode for re-watching cutscenes.