The "7-Day War" -- seven days of nuclear winter -- have caused massive ecological shifts on Earth. The ground splits open, the oceans boil, and all living things on the planet begin to change...including humanity. Scientists create a computer to leave a seed and restore humanity, a computer known simply as "N", which is part of the top-secret Project Yukiwo. What is Project Yukiwo? None who know remain...

Master of Weapon is a vertically-scrolling shooter. The player controls the fighter ship "N", shooting various biological enemies. Power-ups are sparse, with only a handful of upgrades available from item pods. The player can fire in the air, as well as towards the ground to hit land-based enemies. There is also a special weapon which can be changed with power-ups, but also takes time to recharge back to full stock. There are a total of nine "acts", and three difficulty settings.