Two childhood friends, Leo and Shishimaru, made a simple promise to marry each other when they grew up. This innocent vow was met with the adult reassurance that two boys could not marry, forcing both to bury their true feelings deep inside, locked away in a treasure box where they would remain secret and safe. Years later, now in high school and attending different schools, the most precious part of their daily routine remains the walk home together, a quiet moment of happiness they share side by side. Shishimaru, who has spent years repressing his emotions due to social expectations and traditional family pressures, appears strong and unflappable on the outside but is deeply insecure about his feelings for Leo. Leo, who grows up in a more supportive environment, is sensitive and emotionally mature, aware of his heart's desire even if he cannot speak it aloud.

The story's central conflict ignites when Shishimaru sees Leo walking and talking intimately with a girl, leading to a painful misunderstanding that forces his long-suppressed feelings to overflow. Consumed by jealousy and fear, Shishimaru begins to avoid Leo, believing he never had a chance and that his hidden love is wrong. This distance becomes the catalyst that finally compels Leo to act, gathering the courage to follow his heart and confess his love directly to Shishimaru, clarifying the misunderstanding about the girl. What follows is not an immediate fairy-tale ending, but a cautious and realistic exploration of what a relationship between two boys in a society filled with unspoken expectations and subtle homophobia truly means. They begin to date, experiencing the sweet and tender moments of young love for the first time.

However, the story does not shy away from the harsh realities of prejudice. Intolerance and the fear of being seen as different eventually take their toll. Overwhelmed by the pressure and terrified of the social consequences, Shishimaru makes the painful decision to break Leo's heart, ending their relationship. This separation, however, becomes the catalyst for his personal journey. Consumed by regret and loneliness, Shishimaru must confront his internalized fears and find the strength to stand up for who he is and what he wants. The narrative arc follows his struggle to rebuild Leo's trust and his own self-acceptance. The Lion Hearts title reflects this internal battle, symbolizing the courage required to protect the one you love and to be a strong lion in the face of adversity. The story ultimately builds toward a hopeful conclusion, where both boys learn that if they must run away from the world's judgment, they will do so together, having finally found the strength to unlock their treasure box.