One day a mysterious spaceship was sighted above Japan. Suspecting an alien invasion, the authorities ordered to shoot it down. However, to everyone's astonishment, the aircraft turned out to be completely unmanned and abandoned. Shortly thereafter, a teenage girl was found in the nearby forest. She doesn't remember anything except her name, Blon-Win, but apparently there is a connection between her and the strange aircraft. Blon-Win eventually finds out that she possesses unique powers, and joins the 4th Unit, a team of bionic soldiers who combat crime.
The 4th Unit is a Japanese-style adventure game. The emphasis is on advancing the story by triggering events rather than puzzle-solving. The list of commands, however, is fairly long, and includes (beside the standard "Examine", "Talk", and "Go") also verbs for taking objects, using items, affirmative and negative responses during certain dialogues, as well as the unique commands "Fight" and "Rescue". Each of these commands can be combined with a person or an object displayed on-screen by highlighting it.
At certain points during the story Blon-Win and her allies will have to fight enemies. The fights are turn-based and consist of simple menu-based choices of attacks such as punching, kicking, etc.