Synopsis
This issue begins with Steve Rogers' transformation into Captain America via Prof Erskine's super-soldier process, that the US government has organised in the first 4 issues, and ends with Cap's first mission.
This version of Cap's origin contains various standard elements. Rogers drinks Erskine's serum and is bombarded with Vita-Rays. Erskine is killed by the Nazi spy Kruger who infiltrated the project as Agent Clemson last issue. (He was named Kruger in CA#109 and given the alias Clemson in CA#255.) Newly-vitalised Rogers accidentally kills Kruger. The super-soldier process can't be repeated because Erskine didn't commit the whole of his formula to paper.
Steve undergoes a period of training to use his new body, and to master the fighting arts.
Meanwhile government agents have tracked Kruger/Clemson back to the other spy Major Kerfoot alias Prof Hamilton. Kerfoot fakes his own death and is contacted by their unnamed handler. He has transmitted Erskine's incomplete formula, that he copied last issue, to Germany, but he has also written it (or at least some of it) in his journal. The handler kills him, presumably because his cover is blown, and sends the journal to Germany.
Angel has been investigating the death of P.I. Ferret last issue. Despite Ferret's burned down office he has traced the client whose mother was murdered in #2, and followed the leads which led Ferret to Hamilton, whose apartment is now under government guard. Angel has discovered Hamilton was the Nazi spy named Kerfoot, and tries to track down any Nazi activity in the area. Daily Bugle reporter Kellerman (who I think is completely invented here) tells him of the murder of Erskine a few weeks ago. He also mentions Hitler now has his own costumed soldier, the Red Skull.
In France Red Skull is killing villagers trying to get them to betray the Allied fighters who are causing trouble in the occupied territory. These men are Nick Fury, Red Hargrove and the Sky Devils plus John Steele, escapee from the Nazi superhuman Project Nietzsche lab in Frankfurt, all of whom we have been following in the previous issues. But the Sky Devils have been ordered back to England, and Lt Sawyer wants Fury and Hargrove back in the US. Steele stays to continue his activities.
Red Skull's appearance here predates any in CAC or any other Timely comic. But his back-story has been told in several modern comics, the first being Tales of Suspense #66.
We never find out why Fury and Hargrove (nor the Sky Devils) are recalled, though the Adventures of CA limited series does hint that Fury helps train Cap. But we do find out what Lt Sawyer did next in #7, which actually makes that hint more feasible.
Hans Bruder has been moved from the destroyed Frankfurt lab to another Project Nietzsche lab in Berlin, but he has lost all his samples from Steele. He is now working on what they have of Erskine's formula. Also at the lab is a scientist named Burstein, who has a son Noah, who will presumably become the Noah Burstein who empowers Luke Cage amongst others. Presumably with some descendant of Erskine's process.
Back in New York Roosevelt has given Steve Rogers his Captain America uniform after his training is over, and Gen'l Phillips has shown him film of Red Skull. They want Cap as a counter-symbol. Steve suits up as Captain America for the 1st time. He has the historically accurate triangular shield and a helmet not connected to the rest of his costume.
President Roosevelt has been involved in CA's origin since CAC#1. But this is the first time he is said to give Steve his 1st uniform (behind the scenes). He is usually, since CA#255, depicted giving CA his 2nd uniform with the round shield. Gen'l Phillips tells Cap about the Red Skull here (also behind the scenes). This was 1st shown in CA#255, when it is Phillips who gives Rogers the 1st outfit to act as a counter-symbol to the Skull.
Cap has been sent after a nest of Nazi spies at a wharf. Angel has traced the murders of Phantom Bullet in #2 and Ferret last issue to the same group. He watches Cap take most of them out, but the leader escapes. This guy is Kruger and Kerfoot's handler who actually committed the murder of at least Ferret. Angel tries to stop him but gets a flesh wound in the leg. Cap and Angel meet.
This 1st mission is probably based on the (not his first) mission to the Liberty shipyards in CA#255, retold in Adventures of CA and #25 of Brubaker's own CA series. Angel wasn't shown in any of them.
In the midst of all this we see cameos of 2 more early Timely heroes, Thin Man and Hurricane. The stretchy Thin Man, who could effectively go 2-dimensional, only appeared in Mystic Comics #4. But Roy Thomas made him one of the Liberty Legion, and later he was in the modern New Invaders. Technically speedster Hurricane was restricted to a strip in CAC#1-11. But he had previously been in Red Raven #1 as Mercury, and Marvel Universe #7 revealed him to actually be Makkari of the Eternals.