It's the beginning of World War II and Steve Rogers wants to join the army and fight the Nazis. But he's too weak and poor of health. His best friend, Bucky Barnes, is being shipped off and this only depresses him since he can't be a part of the fight. But a German scientist, Abraham Erskine, working for the U.S. government overhears their conversation and approaches Rogers. He's developed a process to create a "Super Soldier" and believes Rogers may be the right candidate, primarily because he's a good person. Steve doesn't have any agenda other than to "do the right thing". In fact, when he's going through boot camp and their commanding officer throws a grenade amongst the soldiers, he selflessly throws himself on the bomb, not realizing it's a dummy.
There's also a woman lieutenant, Peggy Carter, who has an interest in Rogers before he undergoes his transformation. This whole sequence is pretty intense, as he's taken at a hidden lab in New York City and given a series of injections and exposed to "Vita rays". One of the other scientists involved is Howard Stark, Tony Stark's (IRON MAN) father.
The main villain here is Johann Schmidt, the Red Skull (Hugo Weaving of THE MATRIX), who was the first to try out Dr. Erskine's serum, before it was ready. But whereas Rogers was good, Schmidt was bad and it only made him more evil. He runs the Nazi's scientific division, Hydra, but has plans of his own that go way beyond Hitler. He has recovered an ancient, powerful object that is said to belong to Odin (Thor's father) and uses this to power his new machines and weapons, like disintegrator guns.
The final confrontation between Captain America and the Red Skull is not disappointing, and sets the stage for the upcoming AVENGERS movie, as it ends with Rogers being in the present day and meeting Nick Fury.
This is one of the best comic to film adaptations ever. It is exactly what a Captain America movie should be--and it is.