Captain Marvel

1970 — Camp Lehigh, New Jersey. Howard Stark is expecting his first child. Not important now, but the Avengers will time-travel to this exact moment in Endgame, and Tony will meet his father as a young man.

1987 — Hank Pym, S.H.I.E.L.D.'s original Ant-Man, and his wife Janet van Dyne, the Wasp, work covert missions. On one, Janet shrinks past the subatomic limit to disarm a Soviet missile and vanishes into the Quantum Realm. Pym believes her dead, quits S.H.I.E.L.D., and raises their daughter Hope alone and badly.

What is the Quantum Realm, and how do you "vanish into" it? It's a dimension existing below the smallest measurable scale of reality. Shrinking works via Pym Particles, which reduce the distance between atoms. If you shrink past the subatomic limit — "going subatomic" — you fall out of normal space entirely and end up there.

Janet did it deliberately, to phase through a missile's casing and disarm it, knowing she couldn't come back. Hank believed return was impossible.

Scott Lang proves otherwise in 2015, and that single fact is the seed of Endgame's entire solution.

Quantumania (film) later reveals the Realm contains whole civilisations and cities, and that time runs differently there — five hours inside was five years outside.

1988 — Missouri. Eight-year-old Peter Quill watches his mother die of cancer, runs from the hospital, and is abducted from the parking lot by a spacecraft. He is raised by Yondu Udonta, a Ravager, who had been contracted to deliver him to his father, Ego — and who quietly refused, having learned what happened to Ego's other children.

16 December 1991 — Long Island. Howard and Maria Stark are murdered on a country road. It is reported as a car accident. It is not: HYDRA sent the Winter Soldier — the brainwashed, cybernetically-armed Bucky Barnes — to retrieve super-soldier serum from their car. Tony Stark does not learn this for twenty-five years.

1995 — Los Angeles. Captain Marvel.

Carol Danvers is a US Air Force pilot who was caught in the explosion of an experimental engine powered by the Tesseract, absorbing enormous energy. The Kree took her, suppressed her memories, told her she was one of them, and fitted an implant to inhibit her power. She spends six years as a Kree soldier named Vers.

Pursuing Skrulls to Earth, she meets a young, two-eyed Nick Fury, then a mid-level S.H.I.E.L.D. agent, and discovers the truth: the Skrulls are refugees, the Kree are the aggressors, and her mentor Yon-Rogg has been lying to her from the beginning. She destroys the implant, becomes the most powerful being on the board, and leaves Earth to help Skrull refugees find a home.

Two lasting consequences: Fury loses his eye (to a cat-like alien called a Flerken), and he names the "Avenger Initiative" after Danvers's call sign.

1999 — Bruce Banner and Betty Ross begin the gamma research that will eventually produce the Hulk.

Samuel Sterns, a fellow researcher, is involved. Both facts pay off decades later.