Plot
Diabolik and Eva Kant create a plan to target wealthy aristocrat Countess Wiendemar: Diabolik kidnaps Gabriella Bauer, the countess's banker, enabling Eva to impersonate her and steal a collection of valuable ancient coins, ending with Diabolik arriving in a getaway car for their escape. Everything goes according to plan until a gang of armed men suddenly storm the bank, murder Countess Wiendemar in front of Eva, and make off with the coins as part of their haul.
Inspector Ginko is assigned to investigate the robbery, aided by his longtime friend Sergeant Palmer. The two manage to identify the ringleader as Diego Manden, a libertine criminal mastermind who hides behind the facade of a prominent lawyer. Diabolik also learns the identity of Manden, and he and Eva ultiize their disguise skills to follow Manden, learn his routine, and eventually, locate his private mansion. Eager to steal back the coins and believing that Manden is alone, Diabolik tries to break in himself and gets ambushed by Manden's gang.
Waking up in the basement, he is surprised to find Ginko there as well; the inspector admits he had attempted the same plan earlier, but that he expects Palmer to pull a rescue any second. Unfortunately, Palmer is caught, and to Ginko's horror, murdered and dumped in the trunk of an abandoned car. As the police were not informed of the operation, Ginko informs Diabolik that no one else is likely to find them. Realizing that their deaths are certain, Ginko asks for one final favor: that Diabolik share his past so that the inspector may finally know his nemesis.
Diabolik admits that he knows little of his past. He was orphaned as a young boy and raised by a secret organization of seasoned criminals on a remote island, led by a famous thief known only as "King". King was impressed by Diabolik's skills and cunning, and eventually seemed poised to make the young man his heir. However, Diabolik then learned that such hopes were false and that his mentor feared him and was plotting his murder. Diabolik struck first, stabbing King dead and using his newly invented mask technology to take his place, thereby obtaining the resources to begin his criminal career.
Eva, struggling with Diabolik's sudden disappearance, sees a news report featuring Ginko's estranged fiancée Altea di Vallenberg; the broadcast contains a coded reference to the last time the two women crossed paths. In disguise as Altea's former stylist, Eva meets with di Vallenberg, who explains what happened to Ginko and Diabolik. She offers to help Eva out of sympathy, and the two travel to Diabolik's secret lair to craft a new rescue plan.
Manden orders his right-hand man to execute Ginko and Diabolik, but the two work together to overpower their assassin and break his neck. Manden's remaining men are lured outside by Eva in Diabolik's car, which uses bulletproof shielding to distract them as she slips inside and picks them off one by one. Manden tries to surrender, only to be shot with a tranquilizer dart. Eva then finds the two men and reveals herself as Altea in disguise while the real Eva shows up to help free the captives. Just then, Eva double-crosses and subdues both Altea and Ginko.
Waking up, Altea finds that Eva and Diabolik are gone, but that a grateful Eva has secretly returned Altea's Black Panther necklace to her. At Palmer's funeral, Ginko kisses Altea and declares his intention to marry her. The film ends with Diabolik and Eva dressed as an elderly couple to rob an auction, with Eva taking possession of a priceless pink diamond.
