Rai Inami and Guest have been together since they were children. The two shared a precious promise to become heroes together someday. Rai, who was good at tinkering with machines, used those skills to become a mechanic for heroes and eventually became a hero himself. As he drew closer to his dream, Guest was naturally supposed to be by his side. However, starting from a certain day, Guest begins to turn down Rai's invitations repeatedly. "I can't today," "Maybe next time." Even when he asks for a reason, the answers are always vague. While Rai acts cheerful, his loneliness gradually grows. Why is his childhood friend, who used to be by his side as a matter of course, distancing themselves from him? He just can't understand the reason. Even so, Rai couldn't bring himself to hate Guest. Rather, the more time they spend apart, the more he realizes how important their presence is to him. Guest who breaks the promise. Rai who continues to wait. The small misunderstanding born between the two will eventually change their futures significantly. This is the story of a hero who fulfilled his dream and a childhood friend who is disappearing from his side. What lies beyond the promise they exchanged—?