Diabolik and Eva Kant craft an intricate plan to target the wealthy aristocrat, Countess Wiendemar. Their scheme begins with Diabolik kidnapping Gabriella Bauer, the countess's trusted banker, so that Eva can impersonate her. Disguised as Gabriella, Eva aims to steal a collection of valuable ancient coins, which are kept in the countess's bank. The plan reaches a thrilling climax when Diabolik, in a fast getaway car, arrives just in time to facilitate their escape. Everything seems to go smoothly until unexpectedly, a gang of armed men storms the bank, catching everyone off guard. In a shocking turn of events, they murder Countess Wiendemar right before Eva’s eyes, and make off with the coins, completing their raid and leaving chaos behind.
The police, led by Inspector Ginko, are promptly assigned to investigate the robbery. He is assisted by his loyal friend, Sergeant Palmer, and together they begin piecing together the clues. Their investigation quickly points to Diego Manden, a notorious criminal mastermind who, under the guise of a respected lawyer, hides the true extent of his criminal enterprise. Diabolik learns Manden’s real identity and, along with Eva, employs their disguising skills to track him. They manage to follow his regular routines to locate his secluded mansion deep in the countryside. Believing they can reclaim the stolen coins, Diabolik attempts to sneak into Manden’s estate alone, confident that he is unaccompanied. However, his attempt turns into a trap, and he is ambushed by Manden's gang.
Diabolik wakes up restrained in a basement, only to discover Ginko is also there—much to his surprise. The inspector confesses that he had tried an earlier plan to catch Manden but had failed. Tragically, Palmer, who was supposed to help in the rescue, was captured and, in a devastating turn, murdered, with his body dumped in an abandoned car trunk. Since no one was informed of their operation, Ginko admits to Diabolik that their chances of rescue are slim, and their deaths seem inevitable. Before they face their grim fate, Ginko makes a final request: that Diabolik shares his own past, so that he might understand his elusive nemesis better.
Diabolik reveals that he knows little about his origins. Orphaned as a young boy, he was raised on a remote island by a secret society of criminals, led by a legendary thief known only as "King". King recognized Diabolik’s talent and cunning, and was eager to make him his successor. However, Diabolik soon discovers that the trust was false—King feared Diabolik and considered him a threat. In a desperate act of self-preservation, Diabolik kills King using a mask technology he invented, which allows him to assume different identities. He then takes King’s place, securing the means to start his own criminal empire.
Meanwhile, Eva, distressed by Diabolik’s mysterious disappearance, watches a news report featuring Ginko’s former fiancée, Altea di Vallenberg. The report includes a coded message hinting at their earlier interactions. Disguised as Altea’s former stylist, Eva confronts her to understand what happened to Ginko and Diabolik. Moved by her story, Altea offers her help, and together, they travel to Diabolik’s secret lair to devise a new plan to rescue him.
Manden orders Loris, one of his men, to execute Ginko and Diabolik, but the two are able to fight back and overpower their assassin, even managing to outsmart him and end his life. Eva, meanwhile, creates a distraction by using Diabolik’s bulletproof car to lure Manden’s remaining men outside. She skillfully fights them off, one by one, until Manden attempts to surrender. However, he is tranquilized and captured. Revealing her disguise as Altea, Eva unwittingly surprises her enemies when the real Eva arrives to help free the captives. A tense conflict ensues, and Eva turns against Altea, subduing her along with Ginko.
When Altea wakes up, she finds Eva and Diabolik gone. However, Eva secretly returns Altea’s cherished Black Panther necklace as a token of kindness. At Palmer’s funeral, Ginko confesses his feelings and kisses Altea, expressing his hope to marry her someday. The story concludes with Diabolik and Eva, disguised as an elderly couple, pulling off a daring theft at an auction. Eva successfully recovers her signature pink diamond, a treasured keepsake from her past adventures, symbolizing her resilience and unwavering partnership with Diabolik as they continue to outsmart their enemies and chase their elusive fortunes.
