OK, we've watched Back to the Future III (1990), so along with the original and the first sequel, we've seen the whole thing. I wish the last one had been better, but I really just wish there were more of them.
It starts out in 1955 with the usual confusion of people appearing and disappearing in time, we find that 1985 Doc Brown (Christopher Lloyd) has gotten trapped in 1855 in the old West - but he likes it there. But history shows that he was killed a week after he sent that message. So it's back to the ... past!
One thing I learned from Star Trek - in a certain era, all TV shows eventually did a Wild West episode, because the standing sets and costumes are available cheap. I don't know if that was the idea behind BttFIII, but it was almost 100% set in the past. There was no going back and forth, changing the past to change the future. No watching past-you do one thing while future-you undoes it.
Which is too bad. At the start, there's some frantic time travelling that's kind of fun. In this one, the cleverness doesn't come from setting up and resolving paradoxes. It comes from finding excuses to say the name of the movie, or to repeat catch-phrases. I was just saying I would kill Doc Brown if he said "Great Scott" one more time, and then Marty says it.
We do get to see Marty and Biff's pioneer ancestors, and Doc gets a girlfriend (Mary Steenburgen, quite fetching). And in the end, Marty wakes up the girlfriend he left on the porch.
So, it was a fun ride. Not as good as I or even II, which I liked more than some. A good send-off for the franchise.