"Heart o Nusumu no wa Dare da" is a manga series that revolves around the mysterious theft of a valuable artifact known as the "Heart," a gemstone with immense cultural and historical significance. The narrative begins with the sudden disappearance of the Heart from a heavily guarded museum, sparking a citywide investigation. The theft is attributed to a notorious phantom thief, whose identity remains shrouded in secrecy. The thief leaves behind cryptic clues and taunting messages, challenging authorities and rival thieves alike to uncover their true motives and identity.

The protagonist, a young and resourceful detective named Akira, is tasked with solving the case. Akira is known for their sharp intellect and unconventional methods, often clashing with traditional law enforcement. As the investigation progresses, Akira encounters a diverse cast of characters, including rival detectives, skilled thieves, and individuals with personal stakes in the Heart's recovery. Among them is Rina, a cunning and enigmatic thief who becomes both an adversary and an unlikely ally to Akira. Rina’s motivations are complex, tied to a personal vendetta and a deeper connection to the Heart’s history.

The plot unfolds through a series of heists, chases, and intricate puzzles, with each chapter revealing new layers to the mystery. The Heart itself is revealed to possess supernatural properties, tied to an ancient legend that suggests it holds the power to grant immense fortune or catastrophic destruction. This adds urgency to the race to recover it, as various factions seek to exploit its power for their own ends.

Themes of trust, betrayal, and the blurred lines between right and wrong are explored as alliances shift and characters’ true intentions come to light. The narrative maintains a steady pace, balancing action-packed sequences with moments of introspection and character development. By the series’ conclusion, the true identity of the phantom thief is unveiled, along with the Heart’s ultimate fate, resolving the central mystery while leaving room for reflection on the characters’ journeys.