On a night of working overtime, Cang Yu discovers that a fly is actually his biological older brother, who died in his previous life to save him. The brothers venture into the Three Realms to reclaim his human body, but the ordeal leaves behind a dangerous consequence: whenever he loses his temper, he transforms into a half-fly monster. Romance, a manhunt, and an alien fleet follow one after another. To protect Earth, the older brother fights against his own kind, yet ends up killing his brother in this life with his own hands. When an identical bloodline mark reveals the truth about his origins, where does his true home lie—in the human world or among the sea of stars?