Hatsune Miku and Megurine Luka have grown up very close to each other. After an orphan, Kaito, moves into a nearby children's home, Miku and Luka take him under their wing, and the three vow to stay together.

However, as they grow up, their feelings change. Miku finds herself romantically attracted to Kaito. She admits this to Luka, but when Luka informs her that she holds the same feelings toward Kaito, the two could become rivals at any time. However, with both Miku and Luka unwilling to risk their friendship, they vow that neither of them will ever confess their feelings. But will someone break the promise, and if so, at what cost?