You have been assigned a submarine. 
Your submarine has a sonar system that can detect engine noise from enemy submarines, as well as send out a sonar pulse to detect submarines running their engines in stealth mode.
The last know position of a submarine as well as the projected position of the submarine is shown on screen.
The enemy has the same equipment. Your engine noise, sonar and firing a torpedo will give your position away.
Use your wits and you can defeat the enemy and advance in rank.