You start the game with a fleet of 3 submarines. You must destroy all 10 merchant ships in the top row before you can move on to your next mission. Try not to torpedo hospital ships since it's a violation of international law and the consequences can be immediate. Maneuver to avoid exploding depth charges, hostile torpedo fire, and magnetic mines
Since it takes time to reload torpedo tubes, you get only one torpedo at a time in each direction. Your sub has a limited supply of fuel and torpedoes, and must be resupplied often. A supply sub will pass by once in awhile near the ocean bottom and release a trained dolphin carrying fuel and torpedoes. By making contact with the dolphin's supply pack, you can resupply your own vessel. Whatever you do, don't torpedo the dolphin! He has many friends in these waters, and they won't take too kindly to it.