Erik Selvig is studying the Tesseract for S.H.I.E.L.D. when it opens a portal and Loki comes through and steals it.
Tony and Scott fail to take the 2012 Tesseract after the Battle of New York, so Tony and Steve travel to 1970 and take it from Camp Lehigh instead.
Endgame goes back to the very case and the very cube from The Avengers. The first attempt fails and the team has to look for it even further back.