A multi-story, multi-ending love adventure game based on Shin Takahashi's original manga. The player becomes the main character, Shuji, and experiences the last love on earth with the ultimate weapon, Chise.

The purpose of the game is to keep Chise in a happy state and continue to fall in love with her until the last day on earth. Depending on the player's choices and actions, "Chise"'s "happiness value" changes, and "Chise" becomes an existence more than "she" or more than "weapon".

Shuji and Chise are ordinary high school students that start dating after Chise confesses her love. However, Chise is very shy and Shuji is insensitive: neither know how to express their feelings very well, but they do indeed have feelings for each other.

One day, while Shuji is shopping in Sapporo, unknown bombers attack the city in broad daylight. He and his friends run for cover, but notice a fast and small flying object shooting down the enemy bombers. Separated from his friends, Shuji wanders through the wreckage—only to stumble upon Chise; here she has metal wings and weapons—apparently grafted onto her body. She tells him she has become the ultimate weapon, without her knowledge or consent, and that she is seen by the Japan Self-Defense Forces (JSDF) as the last hope for defending Japan.

Will Chise and Shuji's love blossom even if Chise slowly loses her humanity and becomes more of a weapon than a human being?