Night Slave is a side-scrolling action game, influenced in setting and gameplay by the Assault Suits series. The player controls Rei in an armored robot, navigating her through hostile areas. The gameplay is heavy on shooting, the stages consisting mostly on fighting one's way through a level, fighting powerful enemies, mini-bosses and stage bosses.

The player finds power-ups of different categories. Some can restore Rei's health, protect her robot with a shield, etc.; others are used to upgrade main and secondary weapons. Weapons can "level up", depending on how many power-ups were spend to upgrade them. The player can choose when and how to spend the power-ups. Weapons are selected before the beginning of each mission.

A terrorist organization known as Slave Dog poses a threat to the entire world. Steel Fox is the name of an all-female special squad, highly trained in counter-terrorist combat. Assisting them is the independent, state-unbound corporation Neo Logic, which produces assault suits -basically giant robots with their own heavy weaponry, which can be navigated by a skillful pilot. The young female pilot Rei from the Steel Fox is sent on a decisive mission, codenamed "Night Slave" - an assault on the terrorist base.