What 'Avengers: Endgame' tells us about mysteries of time travel

In the climactic battle at Avengers headquarters in 2023, Thanos' blue-hued daughter Nebula confronts - and eventually kills - an earlier version of herself who had traveled forward in time, from 2014. But if she kills that version of herself, then that version cannot return to 2014, in which case she cannot do other things we know that she did from earlier films - make peace with her sister, join up with the Guardians of the Galaxy or fight alongside the Avengers. Even more basically, if the "later" Nebula kills the "earlier" Nebula, there can be no "later" Nebula.