Young Sheldon season 4 subtly lays the foundation to Sheldon's saddest The Big Bang Theory story. The CBS prequel series continues to tackle its titular character's life in Medford, Texas, before he moved to Pasadena and met Leonard and the rest of his friends. So, in an effort to come up with a dynamic sitcom, the rest of his family often plays an important role in every episode; this way, it also allows the spinoff to build on what's previously established in its parent series.
That being said, Young Sheldon is notoriously known for its low regard to continuity. Creatives of the show have been vocal about willing to go against what's previously revealed on The Big Bang Theory for the sake of its offshoot's own storytelling. This is such an odd choice considering that Young Sheldon mainly has the same creatives as the original series, making it easier to ensure that narrative details from the two shows check out with each other. While Young Sheldon can get away with contradicting certain stories from The Big Bang Theory, there are other ones that are too important to Sheldon's arc to be ignored.
One of those not-to-be-missed moments from Sheldon's childhood is his dad cheating shortly before his death. As he sadly told Penny on The Big Bang Theory, he accidentally walked in on George having sex with another woman - the one time he unexpectedly came home early from college, which was the reason he started knocking thrice. At this point in Young Sheldon, he's already in the summer before he officially starts college, and since he didn't spend four years during that time, this tragic experience should be happening very soon. There hasn't been any indication that the show is working its way towards this, but that all changes in Young Sheldon season 4, episode 5, "A Musty Crypt and a Stick to Pee On". As one of the episode's plots, Mary and George Sr. spend the next few days in San Antonio for the latter's coaching conference, but that almost wasn't the case as Sheldon's father deliberately didn't invite his mother to the trip, which was suspicious. In fact, Mary didn't find out about the trip until George's co-worker accidentally let it slip.
This understandably upset Sheldon's mother and resulted in a conflict between the couple. In the end, Mary came with her husband, although they continued to bicker on their drive to San Antonio. Incidentally, George's co-worker and his wife, who rode with them, were the total opposite of the pair. They're all understanding and affectionate - something that initially irritated both of them. As it became clear that Mary and George were in a subtle but tensed argument, the other couple took it upon themselves to try and help.
George's co-worker emphasized the meaning of communication and forced the Coopers to say something genuinely nice about each other, which was a foreign concept to them. Mary earnestly told her husband that she appreciates how hard he works for the family, even sacrificing his personal dreams and wants for them, which has been the case. This is the reason why it's so difficult to imagine him heartlessly cheating on Mary as Sheldon said. But interestingly, after his wife's words, George became teary-eyed, but it doesn't seem like he was just touched by what he heard as he also had a guilty look on his face - as if he's regretting something he's done wrong, and that could be the start of his secret extra-marital affair.
Young Sheldon has already broken The Big Bang Theory continuity one too many times that they can't afford to go against Sheldon's story too about his experience walking on his father cheating. Obviously, there are ways to spin the incident so George doesn't look all that bad, but it's difficult to dispute what he claimed he saw since he has an eidetic memory.
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