It's "kill or be killed" in this first-person murder-mystery. Designed for multiple players, The Ship borrows from the live-action game "Assassin," sometimes played on high school and college campuses. Players take the roles of passengers on a large pleasure cruiser, each becoming both "hunter" and "hunted." For every player, there is one other passenger who is the target for elimination. At the same time, each player is also the target of one other passenger. Players only know the identities their own targets, not the identities of the passengers who are targeting them.

The game's back story has a shadowy benefactor known only as "Mr. X" organizing the deadly competition. Each player is welcomed aboard the ship as a normal passenger, so it is not always obvious who is fair game and who is just along for the ride. The stylized luxury liner setting is intended to evoke the art deco opulence of the 1920s, and its full staff of sailors, servers, and security guards may also factor into the way players execute their objectives. The Ship began as a mod for the original Half-Life before being rebuilt on Half-Life 2's Source engine and distributed as a stand-alone release through Valve's online Steam service.