The SG-1000 port of N-Sub.

The object of the game is to maneuver an on-screen submarine, the "N-Sub," with the joystick and sink the enemy fleet with torpedo fire in the Cobalt Blue Sea. Torpedoes can be fired vertically or horizontally by means of two separate 'FIRE' buttons and 3 torpedoes can be fired in rapid succession by keeping the button depressed. The enemy ships attack the N-Sub with missiles, torpedoes and depth charges. One round is over after the attack by the enemy fleet of twelve ships (No. 1 Fleet Black, No. 2 Fleet Blue) and the loop attack by the enemy destroyer. Enemy attack gains in ferocity with each new round. Bonus points are awarded if the fleet of twelve ships and the destroyer are destroyed. One extra N-Sub is added if players attain high score. The game is over when all of a player's N-Subs are sunk.