THE FIRST SUPERHERO

Johann Schmidt runs HYDRA, the Nazi deep-science division. In 1942 he locates the Tesseract — the Space Stone — in Norway and builds energy weapons from it. He has already taken an unfinished version of a super-soldier serum, which disfigured him into the Red Skull.

Steve Rogers, a physically frail young man from Brooklyn repeatedly rejected for military service, is selected by Dr Abraham Erskine for the perfected serum. It works. Erskine is assassinated immediately afterward, making Rogers the only successful subject.

Where did Howard get vibranium? Wakanda has never been the only source — small quantities leaked out over centuries through theft and black-market trade.

Howard obtained a limited amount through wartime US-government channels, enough for exactly one shield, which is why he tells Steve there isn't any more.

This is a genuine plot hole rather than a mystery. Black Panther (film) establishes that Ulysses Klaue stole a quarter-ton of raw vibranium from Wakanda in 1992 — decades after the shield was made. Marvel has never explained the earlier supply chain and has quietly left it alone.

Rogers is initially used as a propaganda mascot. He goes rogue to rescue captured soldiers — including his childhood friend Bucky Barnes — and becomes an actual field commander, leading the Howling Commandos against HYDRA alongside Peggy Carter and Howard Stark, who builds his vibranium shield.

Bucky falls from a moving train into a ravine and is presumed dead. Rogers destroys the Red Skull's plan to bomb American cities, but has to crash the aircraft into the Arctic to do it. He is frozen, and survives, for nearly seventy years.

The Tesseract sinks into the ocean and is recovered by Howard Stark, who was searching for Rogers.

Characters to track from here:

• Steve Rogers — will wake in 2011 and lead the Avengers
• Bucky Barnes — is not dead. HYDRA finds him
• Peggy Carter — founds S.H.I.E.L.D. and lives a full life; Rogers's unresolved grief over her drives his final choice in Endgame
• Howard Stark — co-founds S.H.I.E.L.D.; father of Tony
• Arnim Zola — HYDRA scientist, captured, and quietly allowed to work for S.H.I.E.L.D. This is a catastrophe nobody notices for seventy years

Peggy in 1946–47, fighting institutional sexism at the Strategic Scientific Reserve and founding S.H.I.E.L.D. with Howard Stark.

Enjoyable, well-made, and entirely optional. Nothing in the current storyline requires it.