SPOILER ALERT: Do not read any further if you have not watched the first four episodes of This Is Us.
This Is Us is not just captivating its audience, it's turning them into detectives.
While its various storylines are stealing the hearts of viewers, the biggest question mark still remains, four episodes in: What on earth happened to Jack?
RELATED: Milo Ventimiglia Talks 'Hopeful, Inclusive' Hit 'This Is Us'
While Rebecca (Mandy Moore) and Jack's (Milo Ventimiglia) life isn't always portrayed as picture-perfect, it's hard to imagine how their love story may have come to an end.
The audience was left with no choice but to ponder this question at the end of the second episode, when the door opened at Randall's suburban home and we saw an older Rebecca standing beside Jack's best friend, Miguel. Tellingly, Randall's young daughters called them "Grandma" and "Grandpa." Pure heartbreak.
EXCLUSIVE: 'This Is Us' Boss Dishes Big, Bold 'Surprises' Still to Come
We took a look at the most popular theories on Jack's mysterious whereabouts, and the evidence:
1. Jack Is Dead
Evidence: Rebecca was still wearing the moon necklace Jack gave her and there was no mention of him on The Big Three's 36th birthday. Let's not forget, Randall, Kevin and Kate all share their birthday with their father. In the premiere, they referred to him in the past tense when they said, "Do you remember what Dad used to say when something crappy would happen to us?"
But how did he die? Fans were quick to recall the second episode when Jack and Rebecca discussed his drinking and how he was "lowering their score" as parents. As touched as we all were when he vowed to change, it sent minds racing that maybe his drinking had something to do with his death.
Another cause? His job. "Jack says as they are leaving the hospital, the car seats are cop approved meaning .... He is a cop which explains his departure... Killed in line duty," a Reddit user guessed.
"I think Jack died of cancer," Facebook user Bryan D. said. "I think it's definitely cancer and that's why Randall is fighting so hard for his biological dad, William, to see more doctors and get more help, he's already lost his adoptive father to the disease. That's also why Rebecca is so certain he will put this before everything else in his life, she already saw Randall go through this once."
Final thought: Does Rebecca's book choice of Stephen King's 1987 psychological horror novel, Misery, in episode four "The Pool" hint at anything in their future?
2. Jack Is Alive, But Suffering From Alzheimer's
Evidence: The family appeared to accept Rebecca and Miguel's relationship and since Randall's kids refer to him as "Grandpa," it suggests this has been their reality for some time now. Is there a chance Jack gave Rebecca and Miguel his blessing (Gasp!)?
Why would he ever do that? "I'm thinking The Notebook," ET staffer Megan Bauer suggested with her mind-blowing theory. "Jack has Alzheimer's and in the last days of his memory he insisted Rebecca move on with her life and since Miguel as Jack's best friend was also devastated they found comfort in each other. I'm patiently waiting for the last scene of an episode where someone says 'We are going to visit Dad' and we see him in a nursing home/care facility and doesn't recognize one of his children. Cue sobbing."
3. Jack and Rebecca Are Divorced
Evidence: We already got a small taste of their marital ups and downs and how Miguel suggested Jack doesn't know how lucky he was being married to someone like Rebecca.
But what really broke them up? From what we have seen, Rebecca didn't tell Jack about meeting Randall's biological father, William. Did he somehow find out? Did it play a part in the demise of their marriage? What other secrets did they keep from each other?
More clues: Randall didn't seem to be the biggest Miguel fan. We could feel the awkwardness when he thanked Miguel for dropping by unannounced. Has he failed to get over the fact that Miguel is the reason his parents are no longer together? Also, why did Kevin get so emotional in the Man-ny scene with his fictional dad, played by Alan Thicke? Fans on social pointed out that it could be art imitating life and he has some unresolved issues with his own father. This theory would point to the possibility that Jack actually left Rebecca, and was in and out of his own children's lives.
While it's hard to believe, no theory is off the table. The teaser for next week's episode may have dropped another huge clue at marriage trouble when we see a younger Rebecca express her doubts over having children. She also ignored Jack when he casually said: "Is now a bad time to mention having another kid?" at the end of "Pool Day."
What do you think happened to Jack? Tell us in the comments below or head over to @etnow on Twitter to share your own theories!
This Is Us airs Tuesdays at 9 p.m. ET/PT on NBC.