The producers of This Is Us hinted awhile back that we'd be spending a lot of time with Jack in Vietnam this season, and tonight's grueling episode certainly made good on that promise. Spotlighting Milo Ventimiglia and set entirely in the past, "Vietnam" finally begins to fill in some gaps when it comes to Jack's military service, and his relationship with his brother Nicky. There was a lot to process, so here are six key things you might have missed.

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1. The soldier who loses his foot was the man Kevin emailed at the end of last week's episode.

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Kevin contacted a man named "Mr. Robinson" last week, asking for information about Jack. Tonight, we met the young Don Robinson (Mo McRae), a soldier who served alongside Jack in Vietnam and shared a rapport with him alongside some good-natured rivalry. As soon as Don started talking about his post-Vietnam dreams of playing major league baseball, you had to know something awful was coming and we're not sure we'll ever recover from the sight of Jack handing Don his own severed foot.

2. The wounded soldier Jack was holding in the season premiere wasn't Nicky.

When Rebecca asks Jack about the war on their first date and he downplays how awful it was by telling her he was 'just a mechanic," we saw a quick glimpse of a bloodied Jack holding someone in his arms. Since the shot came right before he mentioned Nicky, a lot of viewers assumed we were seeing a glimpse of the brother's death scene. But this episode includes the full version of that moment, which sees Jack cradling Don after he loses his foot. Meaning we have no idea at all how Nicky will die, or whether Jack will be there with him when he does, or even whether he's really dead. If there's one thing This Is Us has taught us, it's to question everything!

3. It was Don who taught Jack how to breathe.

One of the most touching recurring motifs throughout the show has been Jack teaching Randall how to breathe through his panic attacks—remember the gut-wrenching scene when Randall did the same for William in his dying moments? Unsurprisingly, Jack did not learn that technique from his own father. Instead, he learned it from Don, who (despite having just lost a limb) takes a moment to comfort Jack as he's about to be medically evacuated.

"You're scared, Pearson," Don tells Jack, asking if he ever gets tired of acting like he's not. "I've been pretending my whole life," Jack admits. In response, Don puts a hand on the side of Jack's face and tells him to breathe, telling him "sometimes we're so scared that we're gonna die, we forget to do the thing that keeps us alive." Jack clearly carried this with him, and years later he taught it to his son. This was so emotional!

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4. Jack was not drafted, but voluntarily enlisted to be with Nicky.

Did not see this coming! Jack had a medical exemption that meant he avoided the draft (more on that in a second), but Nicky's number came up. Jack has always been incredibly protective of his little brother—Nicky only half-ironically refers to himself as "Lois Lane" to Jack's Superman—and so immediately comes up with a plan to get Nicky to Canada to avoid going to war. But en route, Nicky slips out of their motel room in the middle of the night, leaving a note that explains he's not going to flee the draft. "It's my turn to save the day," he writes.

But Nicky does still need saving, at least as far as Jack's concerned. A letter arrives from Nicky indicating that he's not doing well in Vietnam—he's been disciplined, and writes worrying phrases like "I know I'm not getting out of this place alive"—and so Jack has to go and find him. "I just need to be where he is, even if I can't get to him," he tells his doctor. "Even if I can't do anything for him, I just need to be there. He's my little brother. It's my job to take care of him."

5. Jack had a long history of heart problems before his sudden death.

As we all know painfully well by now, it wasn't the house fire that directly killed Jack, but a heart attack caused by smoke inhalation. And as it turns out, there were warning signs. Jack has had tachycardia (an abnormally fast heart rate) ever since his childhood, and it was severe enough to exempt him from the draft. Jack downplays the condition as "harmless" when he decides to enlist, but his doctor is clearly concerned.

6. Jack's father wasn't always a monster.

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This was a pleasant surprise, because Stanley Pearson has been a pretty one-note villain throughout most of the show. On the one hand, this episode offered a lot more painful details of just how abusive hie was to his wife and children, making it clear that his violence is a big part of why Jack is so protective of Nicky. There's also a brutal scene in which Marilyn, Jack and Nicky's mom, greeted the mailman with a very visible bruise over her eye.

"He wasn't always like this," Marilyn tells her sons, after they both intervene to physically protect her from Stanley. And it's true: In a flashback to the day Nicky was born, Stanley is sweet and supportive, offering fatherly advice to Jack in the waiting room. The appearance of Stanley's own father, flask in hand, does hint at some of his demons—still, you have to wonder what on earth happened over the next two decades to transform him into such an awful man. Guess we'll have to wait to find out!

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