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Happy King (Peter Mamakos), a one-time crime boss in Metropolis, who fled to Europe because of Superman's crime-busting efforts, has returned to the city, allegedly to make good on his vow to get even with the Man of Steel. He sends a note to Superman in care of Daily Planet asking to meet and settle their differences. Superman shows up at the appointed time and place, a deserted waterfront building, only to find a machine gun rigged to fire on whoever opens the door -- it's harmless enough to him, though one bullet among the dozens that hit him seems to do a little more harm and less harmless bouncing off him. While he tries to figure out what King could possibly have intended with this stunt, the crime kingpin goes ahead with his plan, convinced by the film secretly taken of the attack that he's on the right track. Professor Meldini (Maurice Cass), a scientist from Europe with a criminal orientation, has developed a theory that Superman, a native of the planet Krpyton, can be rendered helpless and perhaps even killed by being brought into contact with a radioactive isotope of material from his native planet -- kyptonite. And while the naturally occurring kryptonite is so scarce, that he could only put a few grams into that one test bullet, Meldini believes that he can manufacture synthetic kryptonite in sufficient quantity to kill Superman. King prepares to destroy his enemy with a trap, and gets unexpected bait when Lois Lane (Noel Neill) and Jimmy Olsen (Jack Larson) fall into his hands.