The tale follows the master thief Arsène Lupin III and his partner-in-crime Jigen, after a successful casino heist, they discover that the money they've stolen is counterfeit. Desperate to uncover the source, their quest leads them to the small European duchy of Cagliostro. Upon arrival, they witness a runaway bride, Lady Clarisse, being pursued by sinister goons. Lupin recognizes the chase and reflexively aids her, but despite his efforts, Clarisse is captured and her ring taken, hinting at its hidden value. Intrigued by both the counterfeit operation and Clarisse's plight, Lupin sends a calling card announcing his intention to steal her family's fabled treasure, setting the stage for his next grand heist.

Within the fortified walls of Castle Cagliostro, home to the treasure and the criminal operation, Lupin's plans complicate as he encounters Inspector Zenigata, his dogged pursuer from Interpol, who’s also arrived to investigate the counterfeit bills. As Lupin delves deeper into the castle's secrets, he learns of a nefarious plot by the Count of Cagliostro, the estate's ruler, to force Clarisse into marriage and consolidate his power. Thus, Lupin's mission takes a dual nature: to unearth the source of the counterfeit currency and to thwart the Count's dark ambitions—all the while dodging traps, foiling guards, and uncovering the mysteries that lie within the ancient Castle of Cagliostro.