Bummer! The PBS station here is re-running it on Sunday at noon, so maybe there's still a chance you can catch the whole thing. If so, and if you don't want to know what happens at the end, read no further.
Giorgio asks Clara (the Audra McDonald character) to leave her husband and run away with him. She refuses because it would mean she'd lose her child. At this point, we can see Giorgio mentally contrasting her response with Fosca (the Patti Lupone character), who would give up anything for him if he asked her to. Realizing that no one will ever love him the way Fosca loves him (that's for sure), he goes back to the military camp and tells Fosca (who by now is close to death) that he loves her. The next day, he fights a duel with her cousin, who has found the letter Fosca dictated to Giorgio and made him sign, and who accuses Giorgio of "toying with her affections". The cousin is badly wounded in the duel, and Giorgio has a breakdown. In the last scene (some months later), Giorgio is alone, reading a letter from the doctor, who tells him that Fosca died three days after the duel.
That's a pretty bald factual summary. For deeper thoughts on what it all means, see the
New York Times review
I enjoyed it, and thought all the performances were great, but it's so intense I don't think it will become the kind of thing I watch every year (although I intend to keep it on tape for those occasions when I feel some intensity is called for).