Utilizing a modified version of Koei's Dynasty Warriors engine, Crimson Sea features a sci-fi setting where epic battles are primarily fought with laser blasters instead of swords. Alien invaders with the power to change shape are overrunning the five planets in the Theophilus solar system, and it falls to the newly recruited leader of the G-Squad, Sho, to stave off the invasion. Sho and his ragtag band of soldiers embark on over 20 missions that include defeating a certain number of enemies, escorting specific characters to their destination, and retrieving important items. Regardless of the objective, players will find themselves facing unfavorable odds as hundreds upon hundreds of enemies swarm their five-person squad. Between missions (some of which can be completed in the order of your choosing) players are able to acquire new weapons, upgrades, items, and special abilities using the currency accrued on their exploits. Enhancements can be purchased for the two melee and projectile weapons Sho carries, which determine such variables as ammo capacity and recharge rate or the status effects inflicted. An RPG-like upgrade system allows players to increase the effectiveness of weapons in battle through continued use. The more combos unleashed in battle, the greater the experience a weapon will receive. Rounding out the character's repertoire are Neo-Psionic abilities, which grant such powers as health restoration or one of a number of mass-destruction capabilities. Though Crimson Sea offers players direct control over Sho, teammates can only be assigned to specific positions or formations around him. However, doing so requires basic knowledge of their strengths in order to best utilize each character's given talents. A letter ranking is awarded upon completion of a level and takes into consideration the completion time, number of enemies defeated, remaining health, and maximum number of hits achieved. Attaining sufficiently high rankings will reward the player with large monetary bonuses or new weapons and abilities.