'Avengers: Endgame' writers admit time-travel plot was so confusing they had to shoot new scenes to explain it.

Our whiteboard depicting time travel trails in 'Avengers: Endgame'

Warning: Major Avengers: Endgame spoilers below.

Christopher Markus and Stephen McFeely learned about the power of Back to the Future firsthand.

After early test screenings for Avengers: Endgame, the sixth Marvel Cinematic Universe entry the power duo wrote, they found audiences were severely confused by their rules of time travel.

McFeely pointed to the scene in the film's climactic showdown when present-day Nebula (Karen Gillan) shoots the still-villainous, time-traveling version of herself who has arrived with Thanos (Josh Brolin) to wreak havoc. "Much of the audience was expecting that Nebula to disappear. And we're going, 'No, we told you, it doesn't work that way!' So we really needed to underline it," he explained.

"It's a testament to Back to the Future, that everyone who walks into a time-travel movie goes, 'Well, I know how this works.'"

Said Markus: "We would try to imply our rules lightly and people would go, 'So it's Back to the Future.' And we very much needed to be not to be Back to the Future or our movie doesn't make sense, no offense to Back to the Future."

By Kevin Polowy.

Full story at Yahoo News.