Mandy Moore Dishes New Details on 'This Is Us' Season 3 Vietnam War Storyline

Season three of NBC's popular drama This Is Us is set to explore Jack's time in Vietnam, and star Mandy Moore is revealing more details about the upcoming plot.

Fans itching to learn more about Jack Pearson's past are finally going to get answers when the third season of the NBC series ventures to Vietnam, a heartbreaking storyline that Milo Ventimiglio's onscreen wife Rebecca, portrayed by Moore, has claimed is "really powerful."

"It's hard to say stuff without giving anything away," Moore told PEOPLE. "Jack has a whole Vietnam storyline that I think is so fascinating and not often seen, something of this magnitude on network television."

Fans got their first glimpse of Jack's time in Vietnam in a short teaser trailer released in August that showed the character dressed in camouflaged Army war gear and holding a gun as he leaned out of a flying chopper.

"It's really powerful, and it gives us further evidence of who this man was, like what he brought to the table in his relationship, in his marriage, as a father," Moore said of Jack's Vietnam storyline, which is set to be a prominent storyline in season three. "There's a lot about that time in his life that nobody knows about that we'll start to dig into this season, and I think it's really interesting and compelling."

The upcoming third season of the popular NBC drama is set to delve deeper into Jack's past, which has mostly remained a mystery to the audience through the first and second season. It had previously been revealed that the character, portrayed by Ventimiglia, had lost his younger brother while serving in Vietnam, and as Jack's youngest son Kevin travels to Vietnam to learn more about his father's past, more of Jack's time in service will be revealed.

"He's going there — it's a healing thing for him — so he's going to get a lot of information, some stuff that he didn't know and didn't expect. And it's going to jar a few things," said Justin Hartley, who portrays Kevin. "It's not what anyone is expecting. No one is going to see it coming. It's ambitious stuff.... Here's the cool thing about it: At the end of season 3 when you go back and watch the beginning of the series, you're going to [be] like, 'Wow, it all works,' but you never would see any of this coming. I don't know what those writers are eating, but it's brain food. They're brilliant."

This Is Us returns for its third season on Tuesday, Sept. 25 on NBC.