Travelling back to 1999, a "Bio Warrior" named Dan intends to prevent an alien invasion before they take over the world, such is the case in Dan's present. It was Atlus' second NES game in September of 1987 (the other being their first Megami Tensei game). Only released in Japan, a US version was planned but later cancelled close to being finished. Oddly, this version would have been called "Bashi Bazook: Morphoid Masher".
The gameplay is much like other action-adventure games of the time, such as Castlevania. The player moves left or right, jumping on or over platforms and fighting the enemies that are roaming around. The player can enter doors to discover new areas or talk to NPCs, who will provide advice or sell Dan new weapons. There are also innkeepers and various mini-games as well.
The goal of each stage is to reach the boss. The faster the player reaches the boss, the less health the boss will have, giving them ample motivation to move quickly.