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The Mechanical Monsters is the second in the famous Fleischer series of Superman cartoons and contains two notable "premieres" -- the first time Superman uses his x-ray vision and the first time Clark Kent uses a phone booth to change into Superman. In this short, Metropolis is the scene of a series of strange crimes. Giant robots, under the control of the Mad Scientist that created them, are robbing establishments of money and jewels. Naturally, the Daily Planet's top reporters, Clark Kent and Lois Lane, are after the story behind these robberies and the mechanical monsters that are perpetrating them. They arrive at the scene of a robbery in progress at a jewelry store; trying to intervene, Lois somehow gets trapped inside one of the robots. Kent makes the switch to the mighty Superman and follows the robots as they make their way back to the scientist's lair, but he gets waylaid by some pesky power lines. While he deals with this distraction, the Mad Scientist discovers Lois, ties her up and plans to get rid of her by pouring a cauldron of molten steel on top of her. Superman arrives with barely a second to spare, rescues the intrepid girl reporter, makes mincemeat of the robots and brings the Mad Scientist to justice.