Howard the Duck

Twenty-seven-year-old Howard the Duck lives on Duckworld, a planet similar to Earth, but inhabited by anthropomorphic ducks and orbited by twin moons. As he is reading the latest issue of Playduck magazine, his armchair begins to quake violently and propels him out of his apartment building and into outer space; Howard eventually lands on Earth, in Cleveland, Ohio. Upon arriving, Howard encounters a woman being attacked by thugs. He defeats them using a unique style of martial arts. After the thugs flee, the woman introduces herself as Beverly Switzler, and decides to take Howard to her apartment and let him spend the night. The following day, Beverly takes Howard to Phil Blumbertt, a scientist who Beverly hopes can help Howard return to his world. After Phil is revealed to be only a janitor, Howard resigns himself to life on Earth and rejects Beverly's aid. He soon applies for a job as a janitor at a local romance spa. Because of unfair treatment, Howard quits and rejoins Beverly, who plays in a band called Cherry Bomb. At the club where Cherry Bomb is performing, Howard comes across their manager, and confronts him when he insults the band. A fight breaks out, in which Howard is victorious.
Howard rejoins Beverly backstage after the band's performance and accompanies her back to her apartment, where Beverly persuades him to be the band's new manager. The two begin to flirt, but soon after that they are interrupted by Blumburtt and two of his colleagues, who reveal that a dimensional-jumping device they were inventing was aimed at Howard's planet and transported him to Earth when it was activated. They theorize that Howard can be sent back to his world through a reversal of this same process. Upon their arrival at the laboratory, the device malfunctions when it is activated, raising the possibility of something else being transported to Earth. At this point, Dr. Walter Jenning is possessed by a life form from another alternate dimension. When they visit a diner, the creature introduces himself as a "Dark Overlord of the Universe" and demonstrates his developing mental powers by destroying table utensils and condiments. A fight ensues when a group of truckers in the diner begins to insult Howard. Howard is captured and is almost killed by the diner chef, but the Dark Overlord destroys the diner and escapes with Beverly.
Howard locates Phil, who is arrested for his presence at the laboratory with no security clearance. After they escape, they discover an ultralight aircraft, which they use to search for the Dark Overlord and Beverly. At the laboratory, the Dark Overlord ties Beverly down to a metal bed and plans to transfer another one of his kind into her body with the dimension machine. Howard and Phil arrive and apparently destroy the Dark Overlord with an experimental "neutron disintegrator" laser, which has only been forced out of Jenning's body. The Dark Overlord reveals his true form at this point. Howard fires the neutron disintegrator at the hideous beast, obliterating the creature, and destroys the dimension machine, preventing more creatures from arriving on Earth, but also ruining Howard's only chance of returning to his planet. Howard then becomes Beverly's manager, hires Phil as an employee on her tour, and plays guitar with Beverly on stage.

Question: What does the Dark Overlord tie Beverly down to?
Answer: the metal bed