N-Sub puts the player in control of a submarine in the Cobalt Blue Sea. The action takes place on one screen, the player-controlled submarine can move in all directions and must defend itself against an onslaught of enemy ships and submarines by firing torpedoes at them. Torpedoes can be fired vertically (which is the only way to hit ships) and horizontally (to deal with enemy submarines), activated by two different buttons. Pressing the button down allows firing short bursts of three torpedoes each. A round is over after an enemy fleet of twelve ships and a destroyer has completed a fixed amount of loops. Subsequent rounds increase the difficulty by making enemy ships more aggressive.