'This Is Us' Star Michael Angarano Reveals Heartbreaking Reason Jack Lied About His Brother

This Is Us fans finally learned the truth as to what happened in Vietnam that led to the breaking point between Jack Pearson, played by Milo Ventimiglia, and his younger brother Nicky, a troubled, drug-addled medic who was drafted into the war.

Spoilers for season 3 of This Is Us ahead.

On Tuesday's episode, "Songbird Road: Part One", the Big Three took a trip to Bradford, Pennsylvania, to visit their long-lost uncle and search for an answer as to why their dad lied to them and told them he was dead.

"The actual storyline is something that nobody guessed," Michael Angarano, who plays younger Nicky, told Entertainment Tonight on Tuesday. "They were really good at making you think the worst, but it's actually something completely different that's equally as terrible — but not anything anybody thought it was going to be."

In "Songbird Road: Part One," it was revealed that an innocent Vietnamese boy died after Nicky accidentally let a grenade slip and fall in the boat. While the incident was completely accidental, Nicky's attitude toward the war (and the Vietnam natives) and problems with drugs led Jack, who had signed up for the war to search for and care for his brother, to believe that Nicky may have intended harm toward the child.

"The first conversation I had with [co-showrunner] Isaac [Aptaker], he told me that I take a young Vietnamese boy grande fishing. It's a real thing that they did," Angarano said, explaining that war veteran and author Tim O'Brien, co-writer of the episode, experienced a similar situation when he was in Vietnam. "When I heard that that's what happened, I thought, I don't know how we ever are gonna come back from that. I really don't. I don't know that [Nicky] does."

"All of the stuff we shot between Milo [Ventimiglia] and I and the young boy — all the stuff in the Vietnamese village — was really intense. It didn't feel like we were making This Is Us, it felt like we were shooting something different. It felt like a war movie starring Milo Ventimiglia," he added.

The boat explosion was mirrored by an equally emotional one from Jack, who was fed up and "done" with trying to help Nicky. As a result, Nicky was shipped off for psychiatric reasons and eventually discharged as a medic.

"There's a moral line in Jack's heart that he's always lived by and this just crosses that line. When he got there, he saw Nicky was having a really hard time and Nicky's own struggles and demons were challenging for Jack because Jack is somebody who, if he can't help you that's a reflection of his inability to help you," Angarano said. "So he felt really helpless with Nicky, but when it crosses that line and it involves a young boy's life, I think that was it for Jack."

Years after the war, Jack finally paid a visit to Nicky in 1992 for the first time after he sent a series of postcards to his older brother's work and home addresses. But even then, Nicky didn't tell Jack that the little boy's death was an accident.

"I think there's a part of [Nicky] that thinks even though he didn't intend to kill that boy, it was his fault," Angarano said, adding that he didn't think it would have made a difference if Jack knew the boy's death wasn't meant to happen. "I think Jack is more disappointed in the fact that Nicky allowed that to happen, accident or not an accident."

In October, series creator Dan Fogelman promised that a resolution to Jack and Nicky's strained relationship was coming sooner rather than later.

"It's a big story. It's surprising. It's fair to say that clearly, something broke in Vietnam for Jack as it relates to his brother, and I think people will be satisfied with what we're doing there even if they don't always like it," Fogelman told ET in October. "It takes place in a compressed period of time that was a defining period of time for Jack as it relates to his brother. I think it would be borderline impossible to guess where it's going because it's very specific."

"It's comparable to some of our biggest stuff in that it's going to feel really massive, in terms of what it does to Jack," he said at the time. "Jack buried a portion of his life when he came back from the war. What we're going to explore this season is what he buried."

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