Tetsuya Wakatsuki, a young photographer, records an alien invasion that brutally wipes out the human population. After a series of mishaps force him against the enemy commander, Tetsuya is knocked out. To his surprise, he regains consciousness on a spaceship, where the alien commander, Princess Kahm of Saint Evascuraze, reveals that she has taken a liking to him. She now wants him to become her husband! Although initially hesitant to accept the proposal, Tetsuya soon realizes that marrying Kahm could potentially save the rest of humanity from certain doom. To stop the invasion, he decides to marry the alien princess—even if it goes against the wishes of her father, the emperor.