Magneto is a Mutant with the power of controlling metals. While he mostly appears in X-Men books, he has also faced Captain America a number of times.
Both Captain America and Magneto read a newspaper ad about a Mutant who needs shelter from people or agencies equipped to help him. They converge to find the Mutant, a minuscule being called Mister One, who can fit inside a wristwatch. Magneto needs a Mutant of that same size to unravel the secrets of a small spaceship he has found, so he sends his new Brotherhood (the version eventually known as Mutant Force) as a diversion while he kidnaps Mister One. Once inside the ship, Mister One activates the self-destruct mechanism, therefore denying its secrets to Magneto.
At some point, most villains conspire against the Avengers and other heroes in the "Acts of Vengeance", where Magneto, a survivor from Nazi concentration camps, sees himself working alongside the Red Skull. Near the end of this conspiracy, Magneto confronts the Skull about his time as a Nazi. Magneto defeats the Skull's guards, shields himself from his Red Dust, defeats his robots and magnetically bends the rails Red Skull has as an escape route. Magneto confines the Skull in a former fallout shelter, 20 feet underground, with the ladder removed and with only water and air.