One Can Have Her: A Game Noir is a roleplaying game about hard decisions and sinful actions in a morally gray world. Inspired by the film genre, the game is played in black and white, with colors replaced with descriptors. The PCs take the role of men in the midst of trouble, seduced by a manipulative and chameleonic Everywoman only one of them can have.

The winning conditions are framed in a variation on the Prisoner's dilemma: If one PC rats out everyone else to the police Chief, he gets the girl and fulfills his mission; if two or more rat out each other, everyone ends up dead or in prison. All out-of-character knowledge is considered in-character knowledge, and players are encouraged to add elements to each other's PCs. All characters have an archetype, a drive, and a dream woman. Players have bonus (Coolness) cards they can reward to other players, which turned into (Opposition) cards the GM can use against the player when used.

The game is structured around four to seven scenes. The first scene establishes the character's drives, and the next two scenes involve following that drive as the situation worsens. The remaining scenes are similar, but the players are given the option of ratting on the other PCs -- which is worked into the scene. A player narrates each scene in voice-over style, adding any elements desired to the game.

Each PC is involved in a single conflict in each scene. Both the GM and the player decide on the two options (stakes) before the conflict, which are resolved by playing three cards plus any additional bonus cards. Each of the three cards represents an exchange in the conflict, and the side with the most successes in each exchange (red cards are successes) wins and gets to narrate the results. The total number of successes in all three exchanges determines the winner of the scene. Each player narrates their own epilogue.