"That's an interesting dynamic of falling in love with somebody but them not completely revealing who they are," Mandy Moore told PEOPLE

Meet the Pearson family member you didn’t know was still alive — and could possibly change everything on This Is Us.

The Big Three, as well as fans and audiences, were shocked to discover that Jack (Milo Ventimiglia) had lied about his brother Nicky’s existence by previously explaining that their uncle died in the Vietnam War.

In Tuesday’s episode, titled “Songbird Road Part 1,” Kevin (Justin Hartley), Kate (Chrissy Metz) and Randall (Sterling K. Brown) go on a six-hour road trip to Bradford to track down now-70-year-old Nicholas Pearson (Griffin Dunne) after Kevin discovered a post-war postcard from Nicky to Jack.

Credit: Ron Batzdorff/NBC; NBC

A big question from the mid-season cliffhanger was answered as viewers watched the aftermath of the loud explosion in Vietnam, where Jack hurriedly jumped in the lake to rescue younger brother Nicky (played by Michael Angarano) following his fishing accident involving hand grenades and the death of a young boy.

Turns out that incident caused a giant rift between the Pearson brothers. In a flashback scene, Jack angrily yelled at Nicky, saying, “He was just a kid. I’m done, I’m done!”

Credit: Sterling K Brown/Twitter

Jack assumed that Nicky’s animosity towards the Vietnamese villagers (he pretended to shoot them with his hand) led to the innocent boy’s death, thus leading to Nicky’s discharge due to “psychiatric reasons.”

As Nicky later explained to his nephews and niece, “That’s why [Jack] had two lives… He walked away from the first one.”

THIS IS US -- "Songbird Road: Part One" Episode 311 (screen grab)CR: NBC
Credit: NBC

Even when Jack visited him and his Songbird Road trailer in 1992 — six years before he died of a heart attack caused by smoke inhalation in the house fire — Nicky was unable to explain his side of the story.

However, Nicky knew he was an uncle to Kevin, Kate and Randall as Jack proudly showed him a family photo of the Pearsons. “I have a good life,” Jack told Nicky after his brother asked if he ruined his life.

“You have a nice house,” Nicky also told Jack, hinting that he had visited the Pittsburgh home once before.

This Is Us - Season 2
Credit: Ron Batzdorff/NBC

Not only did Jack keep his kids away from their only uncle, the Pearson patriarch also kept his wife Rebecca (Mandy Moore) in the dark.

Moore, 34, spoke with PEOPLE about the revelation, sharing that the Nicky storyline was among the plot twists that surprised her the most this season.

Speaking of the “complicated nature” of Jack, the actress said the repercussions of his secret could play out in later episodes.

“He’s a veteran and we’re going to delve into his life in Vietnam, what he saw and what he experienced. There’s so much of it that he chose not to share with Rebecca and his family that profoundly affected him and changed him,” Moore explained.

“I think it’s a part of him that he really doesn’t show to anyone else. That’s an interesting dynamic of falling in love with somebody but them not completely revealing who they are. It’s tricky territory,” she said.

And her character Rebecca said it best: “But now I want the truth.”

THIS IS US -- "Songbird Road: Part One" Episode 311 (screen grab)CR: NBC
Credit: NBC

At the end of the episode, Kevin lead the Big Three to not make the same decision Jack chose over a decade before. “I can’t leave him like Dad did,” Kevin told his siblings. And good thing he did go back: the trio found Nicky contemplating death as he had a gun close by.

“I never got to tell him. It was an accident,” Nicky said, with his eyes welled.

As Kevin looks to connect with his long lost relative, will Rebecca be the next Pearson family member to meet Nicky? And will they start to question: How much did we really know about Jack?

This Is Us airs Tuesdays (9 p.m. ET) on NBC.

If you or someone you know is considering suicide, please contact the National Suicide Prevention Lifeline at 1-800-273-TALK (8255), text “help” to the Crisis Text Line at 741-741 or go to suicidepreventionlifeline.org.