Recently, an alien race called the mutons has been invading the solar system Theophilus. Like swarming bugs, the mutons threatened the very existence of the solar system. Enter Sho, a bounty hunter who uses a special weapon that can serve as a blade and as firearm at the same time. Starting as a subordinate, Sho quickly becomes the leader of a special squadron of soldiers that will use all its power to stop the muton invasion.

Crimson Sea is a mission-based action game set in a distant future. In some cases, you can choose yourself the order in which you want to tackle the missions. Some of the missions have a time limit. Sho will encounter hordes of mutons and battle ferocious muton bosses. He can find and attach various upgrades to his custom weapon that allow new attacks. He also uses "neo-psyonic powers", which work pretty much like magic spells. There are slight role-playing elements in the game: it is possible to buy weapons, weapon upgrades, and items in stores, and weapons can gain experience points when continuously used in battles.