Milo Ventimiglia as Jack Pearson in "This Is Us". Picture: Screengrab/YouTube

Warning: This post contains spoilers for Season 3, Episode 4 of "This Is Us."

A good rule to follow when watching "This Is Us" is never assume you know everything.

We thought we already knew all that we needed to know about how perfect father/husband Jack Pearson died. Last season, the NBC drama finally revealed the big mystery: Jack (Milo Ventimiglia) died after going back into a house fire, inhaling tons of smoke and suffering cardiac arrest. A "widowmaker's heart attack" is how the sudden death was explained.

Until that point, the show hadn't given us any details on Jack's health, other than his struggles with alcoholism. But Tuesday night offered another layer to the story, during an episode entirely devoted to Jack's time in Vietnam, his relationship with his brother and growing up in an abusive household.

As a young adult, Jack goes to his family doctor and explains that he has to get cleared to enlist so he can protect his little brother, Nicky, or at least just be close to him. The doctor tells Jack he has tachycardia, which is an abnormally fast heart rate, and the condition has apparently prevented Jack from getting drafted like his brother. So the doc gives him a trick to get medical clearance involving an ungodly number of push-ups.

And Jack apparently had this heart condition since childhood. In another scene, he's just a little boy when his mother notices his heart is beating incredibly fast. It always does that, he reassures her (as if that's OK! It's not OK!).

This week's episode also added another wrinkle, by giving us the origin story of Jack's patented cradle-Randall's-face-in-my-hands-and-tell-him-to-breathe trick to calm him down. It turns out Jack's war buddy, who had just lost a leg, used it to calm Jack down.

Up until now, we didn't know anything about Jack's time in Vietnam, let alone his health.

The revelation about his heart prompted many more feelings that need to be processed.

Did Jack's heart condition have anything to do with his death, several years later? It seems very likely. According to the Mayo Clinic, "in some cases, tachycardia may cause no symptoms or complications. But if left untreated, tachycardia can disrupt normal heart function and lead to serious complications," and that includes sudden cardiac arrest or death.

You can bet that Jack never got his condition treated. He never even really told his wife or kids about his time in the war (as far as we know).

But if we've learned anything from "This Is Us" so far, we shouldn't rest easy thinking we've got the full story.