Four years ago, the small island of Balderia was attacked by a mysterious creature with the ability to take any form (except that of a human). Captain Mikan of the Balderian Navy defeated the creature, and peace returned for a time. However, a fishing boat has recently disappeared. Newly promoted Admiral Mikan, realizing the creature has returned, has outfitted the submarine Nautilus to defeat it once again, along with the humans that have captured it and hold it in their underwater complex.
The game is a horizontal-scrolling shoot-'em-up set underwater, where the player captains the submarine Nautilus through six levels destroying enemies. The submarine moves in any direction with the D-pad. It is equipped with two weapon systems: missiles that are launched in an arc above the submarine with and torpedoes that are fired straight ahead with . Both weapons have rapid-fire and can be fired simultaneously. The torpedoes are upgraded to lasers in the last stage.
The Nautilus can sustain three hits from enemies and hazards (such as bubbles). The Nautilus bounces off terrain harmlessly. After it has been hit twice, it flashes to indicate that the next hit is fatal. If it is destroyed, the player loses a life. If the player runs out of lives, the game can be continued from the beginning of the last stage.
After completing the game, the player can play it again at a higher difficulty.
Shoot to release an item.
Upgrades the attack speed of the submarine's weapons.