The plot is basically this: It's 1964, and Firth, an English professor at a small college, is grieving the passing of his partner of 16 years. Moore is the best friend, and the two of them have a history as well. But, basically, it's a movie about grief.
Actually, it can be said better than that: the movie is about many things (being gay in 1964, for example), but it is really about this man moving through his grief: how he does it well, and how he maybe doesn't do it as well as he should.
One of his students in the film is played by Nicholas Hoult, who most of us probably remember from "About a Boy" (2002). And he does a fine job, but the really remarkable thing is that, in seven years, Nicholas Hoult went from this:
Yep. There's our boy Nick.
So, see "A Single Man". It's well-done, it's stylistic, it's totally gorgeous, it's sad, and you'll get to sit there now going, I can't believe this is the same kid who sang "Killing Me Softly" for his mom at the school talent show.