Oh, man, I feel bad now, I didn't mean to spoil that for you.
Anyway, I'll try to sum up the majority of Nate's arch from this past season for you if you want....
DON'T READ THIS IF YOU PLAN ON WATCHING IT, THOUGH...
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OK, well, at the beginning of the season, Nate and Brenda get married, but right after they do, Brenda loses her baby (she was pregnant). She gets pregnant again, but Nate, as usual, is starting to get scared of the normalcy of his life and starts questioning whether he's turning into his father, etc. He begins attending a Quaker church with George's (Ruth's husband) daughter. In one of the last episodes of the season, he goes to her house to pick her up and they end up having sex. As he is getting dressed, he starts complaining that his arm is numb, then he just collapses and that's the end of the episode. In the next one, he's getting surgery at the hospital while most of the family waits. The surgery is a success, and everyone visits Nate. He tells Brenda he doesn't think the marriage is going to work out. Everyone leaves except for David, who wants to spend the night. He falls asleep and when he wakes up, Nate is dead.
He had another one of those AVMs in his brain, that was what killed him. And that's all I'm telling you, because I feel bad enough just ruining this much for you. If you were a fan of the series, please do yourself a favor and at least watch the last episode, I'm still haunted/moved by it, it just sums up the entire series perfectly.