'This Is Us': Kevin and Cassidy's Dramatic Moment Adds Fuel to Popular Season 4 Theory

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Warning: Spoiler alert! Do not proceed if you have not watched Tuesday's episode of This Is Us.

Did This Is Us just confirm a popular fan theory?

After forming an unlikely friendship, Kevin Pearson (Justin Hartley) and military vet Cassidy Sharp (Jennifer Morrison) made a huge leap forward in their burgeoning relationship on Tuesday's episode, aptly titled "The Club." For the early part of the season, Kevin and Cassidy danced around their mutual attraction (even though it was apparent from the get-go the immediate sparks between them), mindful of the fact that crossing that line could be disastrous.

When they met outside of a therapist's office, both were trying to get back on the right track -- Kevin, fresh off a breakup, a recovering addict and a working actor; and Cassidy, a wife and mother whose post-war PTSD saw her estranged from her husband. (Kevin even tried to reunite Cassidy with her hubby!) But everything changed between the two damaged souls in the latest hour of This Is Us.

On the way back to his fancy trailer with his lunch date Donna, Kevin noticed Cassidy waiting outside his door. When Cassidy uttered the words, "I thought you'd be alone," Kevin got the hint and sent his date on her way. As the two entered his trailer, Cassidy couldn't help but fire back one of her signature quips, "You interviewing mothers to bear your children?" After a flirty exchange, Cassidy revealed the real reason she wanted to talk to Kevin: She met with her estranged husband, Ryan (Nick Wechsler).

The meeting started out OK, Cassidy recalled, but the moment he started being critical, that's when things took a turn for the worse. "We started arguing and I lost my temper," Cassidy shared, vulnerable in that moment. "I told you it was a bad idea. I don't blame Ryan. I wouldn't want to be with someone as screwed up as me."

But Kevin doesn't see Cassidy that way. "You've seen things that 99 percent of us, we can't even imagine. Yeah, you've made some mistakes along the way, but you're trying. That doesn't make you unlovable, it makes you human," he said with genuine concern for his friend. In the heat of the moment and with both their guards down, Cassidy closed the distance between her and Kevin for a passionate kiss. But just as quickly as it happened, Cassidy pulled away and apologized for shattering the "friend zone."

As she got ready to bolt, Kevin stopped her from leaving and instead kissed her again, leading to... well, you can probably imagine what it led to, especially after it was revealed later on that Kevin and Cassidy spent the night together. Their reactions, following their intimate evening, left much to be desired. What does this mean for Kevin and Cassidy now that they've crossed into "more than friends" territory? Are they going to give things a shot as a couple?

Whatever this means, it adds more fuel to the fire to the popular season four fan theory that Cassidy could be the mother to Kevin's future son, whom viewers briefly met in the capper in the season three finale. Fans have been warming to the idea as the season has progressed and after Tuesday's episode, it's very hard to bet against it... until This Is Us pulls another major twist.

See the biggest theories below.

Until it's proven otherwise, it seems things are starting to move in a romantic direction between Kevin and Cassidy. Time will tell if they're the real deal or if this is one of This Is Us' patented misdirects.

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

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