You are the commander in charge of three mini-subs. During your mission, you soon discover that the area you were sent to scout is a dangerous one, teeming with several innocent-looking creatures-schools of fish, manta rays, starfish, jellyfish, and an occasional "Nereis," a marine creature which excretes a paralyzing poison. In addition to these native aquatic creatures, there is an enemy navy destroyer which fires depth charges with your submarine as its target.