"Joker" is a manga series set in a world where supernatural phenomena and criminal investigations intersect. The story revolves around the Joker Investigation Team, a specialized unit tasked with solving cases involving mysterious and often otherworldly occurrences. The team operates under the guidance of the enigmatic Professor, who provides them with cryptic clues and advanced technology to aid in their missions.

The protagonist, Kazuki Mikage, is a young man with a tragic past who becomes the leader of the Joker Investigation Team. Kazuki possesses a unique ability to see the "shadows" of people, which allows him to perceive their true intentions and emotions. This ability proves crucial in unraveling the complex cases the team encounters. Alongside Kazuki is a diverse group of team members, each with their own distinct skills and backgrounds. These include Rei Hino, a skilled martial artist with a strong sense of justice; Akira Kogami, a genius hacker with a penchant for solving puzzles; and Yui Kanzaki, a former police officer with a deep understanding of criminal psychology.

The narrative unfolds through a series of episodic cases, each presenting a new mystery that the team must solve. These cases often involve supernatural elements, such as curses, ghosts, and ancient artifacts, which challenge the team's investigative skills and force them to confront their own fears and insecurities. As the series progresses, the team uncovers a larger conspiracy tied to the origins of their abilities and the true nature of the Professor's involvement.

Throughout the series, the characters' personal histories and relationships are gradually revealed, adding depth to their interactions and motivations. Kazuki's past, in particular, is a central focus, as it is closely linked to the overarching mystery of the series. The team's investigations lead them to confront powerful adversaries, including rogue agents and shadowy organizations, who seek to exploit the supernatural for their own gain.

The manga combines elements of mystery, thriller, and supernatural genres, creating a compelling narrative that keeps readers engaged. The intricate plotlines and well-developed characters contribute to the series' appeal, making it a notable entry in the realm of supernatural detective fiction.