Mandy Moore Says ‘This Is Us’ Episode 5 Will ‘Destroy People’

"You'll find out more as to what happens to Jack and Rebecca and why in 2016 they're no longer together," Moore told THR. "What happens to them, what breaks them apart... it's pretty heartbreaking... I was pretty devastated to see they don't end up together."

The big reveal will explain why Moore and Ventimiglia's characters are not together in the present day and, presumably, why Rebecca is now with Miguel (Jon Huertas). The revelation will ultimately change Kate's relationship with her boyfriend as well.

On this week's episode, Mandy Moore's character will also go back in time to the late 1970s, before she conceived triplets. In a flashback scene, fans will find out how the babies were conceived after Mandy's character says she doesn't want children at all. In addition, the family's fandom for the Pittsburgh Steelers will play into the storyline.

In just four episodes, This Is Us has been an emotional roller coaster with several big twists that viewers didn't see coming. Show creator Dan Fogelman told E! News that writers have "carefully mapped out" the gut-wrenching 18-episode story arc for Season 1.

While her character's fate has already been planned out, Moore told Entertainment Weekly that she was stunned to find out that Rebecca's present-day husband was not Jack in the second episode.

"The fact that she would be with somebody else... feels like a betrayal in some way," Mandy said earlier this month.

"I'm rooting for Jack and Rebecca. I want to know their story, I want to know where that disconnect is, because I just love them so much from the first episode. I'm anxious to see where they go, what this journey is… And I'm excited, too, that we're not married to having that storyline in any sort of real time, because we're jumping around."