Arsène Lupin III, a master thief, and his partner, Daisuke Jigen, find themselves in the small European principality of Cagliostro after a successful heist. Upon discovering that the money they stole is counterfeit, Lupin recognizes the bills as part of a larger conspiracy tied to the Cagliostro family, a powerful and corrupt dynasty. Intrigued, Lupin decides to investigate, leading him to the enigmatic Castle of Cagliostro.

The story takes a dramatic turn when Lupin rescues a young woman, Clarisse, from a group of armed pursuers. Clarisse is revealed to be the princess of Cagliostro, forced into an arranged marriage with Count Cagliostro, the ruler of the principality. The Count seeks to exploit Clarisse’s lineage to unlock the secrets of the Cagliostro treasure, rumored to be hidden within the castle. Lupin, driven by a mix of curiosity and a desire to protect Clarisse, devises a plan to infiltrate the castle and thwart the Count’s schemes.

As Lupin and Jigen prepare for their mission, they are joined by their longtime associates: Goemon Ishikawa XIII, a master swordsman, and Fujiko Mine, a femme fatale with her own agenda. Together, they navigate the castle’s intricate defenses, including traps, guards, and the Count’s loyal enforcer, Inspector Zenigata, who is determined to capture Lupin. The narrative unfolds with a series of daring heists, narrow escapes, and confrontations, blending action, humor, and intrigue.

The Count’s true motives are gradually revealed: he seeks to use the ancient Cagliostro treasure, a mechanism capable of producing flawless counterfeit money, to consolidate his power and destabilize global economies. Lupin’s efforts to stop him culminate in a climactic showdown within the castle’s depths, where the treasure’s secrets are unveiled. The resolution hinges on Lupin’s ingenuity, the loyalty of his allies, and Clarisse’s courage as she confronts her destiny.

The narrative concludes with Lupin and his team successfully dismantling the Count’s plans, ensuring the safety of Clarisse and the principality. The story balances its adventurous tone with moments of emotional depth, particularly in Lupin’s interactions with Clarisse, highlighting themes of freedom, justice, and the enduring allure of mystery. The Castle of Cagliostro stands as a self-contained tale within the Lupin III universe, showcasing the characters’ dynamics and Lupin’s signature blend of charm, wit, and resourcefulness.