Legendary thief Diabolik hatches a daring plan to lift a priceless diamond from heiress Eva Kant, who is dating the deputy minister of justice, Giorgio Caron. To pull off the job, he uses one of his signature masks to impersonate his greatest foe, Ginko, and tricks Eva into revealing which hotel she will stay at. He fashions another disguise as a member of the hotel staff and quickly locates the hidden safe where the diamond is kept. Yet, when he sets eyes on Eva, something unexpected happens: a magnetic attraction begins to complicate his usual cool, calculating plan.
Diabolik sneaks into Eva's room to steal the diamond, but Eva walks in and he must hold her at knifepoint. Eva calmly reveals that she doesn’t have the diamond, having already sold it to a dealer in South Africa, and Diabolik departs with frustration and unfinished business gnawing at him. The next day, he returns and unmasks himself in front of Eva; disgusted by the lecherous Giorgio, she begins an affair with him, drawn by a dangerous allure and a momentary sense of freedom. Unbeknownst to Diabolik, his girlfriend Elisabeth, Elisabeth Gay, accidentally uncovers his secret hideout, adding a layer of complication to a already tangled web of loyalties and lies. Ginko is alerted and a sting operation is set in motion, culminating in Diabolik's arrest.
At the courtroom, Diabolik is convicted and sentenced to be beheaded by guillotine, a brutal fate that starkly contrasts with his suave, almost theatrical swagger. When Eva learns of his fate, she uses Giorgio’s influence to arrange a clandestine meeting in Diabolik's cell. Once the guards have left, Eva incapacitates Giorgio and Diabolik interrogates him to learn where a file—containing sensitive information that Eva has used to blackmail her past—is kept. In a bold move, he dons a mask of Giorgio's face and, slipping into Giorgio's clothes, makes a dangerous escape, while a drugged Giorgio is executed in his place. Ginko pieces together the deception in time to stop Diabolik from retrieving the file; another officer ends up with the incriminating document.
Needing money to flee the country, Diabolik sets his sights on a bank holding expensive jewelry and watches. Eva plays a crucial part by impersonating a wealthy widow, Ms. Morel, Claudia Gerini, to rent a vault and study the bank’s security. With this knowledge, Diabolik plots a high-stakes heist that involves flooding the bank and using diving equipment to reach the treasure. Ginko, leveraging his understanding of Diabolik’s techniques, traces the planned escape route and briefly corners them at gunpoint before Eva distracts him, allowing the lovers to slip away aboard a motorboat.
Time passes, and the duo finally wind up on a luxurious yacht. To seal their uneasy alliance, Diabolik presents Eva with the diamond; in a decisive act of defiance against a life weighed down by secrets, Eva hurls the gem into the ocean, signaling that she no longer wishes to be haunted by her past.
