The year is 2037. The police forces of the world have been disassembled due to their incompetence at stopping the ever-rising waves of crime. Instead, private security companies have taken upon themselves to protect citizens or entire companies from criminals for a fee. One of the best-known security services in the city of Freeport is HardCorps, under the command of Colonel John Blade. A large-scale bank heist involving a hostage situation, apparently perpetrated by a well-known gangster Mancini, attracts the attention of HardCorps, and John is summoned to the crime scene. Further investigation reveals the existence of a powerful new drug called the U4, and possible ties to a multi-national biotechnology corporation known as SiNtech, which is guided by the beautiful and enigmatic Elexis Sinclaire. With the help of his colleagues from HardCorps, John will have to avert the greater threat and save the city.
SiN is a first-person shooter that uses a modified version of the Quake II engine. John Blade is armed with a rather standard FPS arsenal, starting with a default bare knuckles and a pistol, and ending with heavy guns such as rocket launcher and plasma rifles. The game is notable for being the first FPS that had missions exclusively based on stealth (i.e., the game is automatically lost if the hero is detected). The game incorporates realistic damage effects: headshots are instantly effective, while shooting an enemy in the knees will merely hinder his movements. SiN also includes the option of hacking computer terminals to disable security.
Cutscenes are employed to advance the plot between the levels. At certain times, the outcome of a mission will be different depending on the player's actions during it. Taking a cue from Duke Nukem 3D, SiN features a wise-cracking main hero, humorous "easter eggs", and environments with a high degree of interaction.