Black Widow

Set immediately after Civil War, while Natasha is a fugitive. She reunites with her fake Soviet sleeper-cell family from childhood: sister Yelena Belova, "father" Alexei Shostakov / Red Guardian, and "mother" Melina Vostokoff. They destroy the Red Room, the programme that trained and chemically conditioned Black Widows, killing its director Dreykov. His daughter Antonia, whom Natasha believed she had killed and who was in fact turned into the mind-controlled Taskmaster, is freed.

What is a "Black Widow," exactly? Girls — often trafficked or abducted as children — are put through the Red Room and trained as assassins, forcibly sterilised, and later chemically conditioned so they physically cannot disobey. Natasha escaped and defected via Clint Barton, who was sent to kill her and chose not to. This film is her freeing the remaining Widows with a chemical antidote.

Taskmaster is Antonia Dreykov, the director's daughter. Natasha blew up a building to kill Dreykov and believed she'd killed Antonia too. Antonia survived, disfigured, and her father installed technology letting her instantly copy any fighting style she observes. She's freed at the end.

Valentina Allegra de Fontaine, who appears in the post-credits, is a senior US intelligence figure who recruits damaged, disposable people for deniable work. By 2025 she is Director of the CIA. She recruits Yelena here and John Walker in The Falcon and the Winter Soldier.

This is the origin point for four current Doomsday roster members. Yelena, Alexei, Ghost (from elsewhere), and Taskmaster all route through the Thunderbolts, and Yelena in particular becomes one of the most important characters in the current phase.

The post-credits scene takes place after Natasha's death, years later — Yelena at her grave, being pointed at Clint Barton by Valentina Allegra de Fontaine. Note that if you watch chronologically: this scene doesn't belong here.