At a park near the palace, the two murderers are joined by a third, which is a little fishy. He says Macbeth sent him, but the First and Second Murderers didn't seem to be expecting anyone else. Check out this fun blog post for some theories about who the Third Murderer may (and may not) be.
Banquo and Fleance approach on horseback and dismount to walk the mile to the palace, as usual. Conveniently, they have a torch—good for seeing by.
Banquo starts up with a friendly "it looks like rain" conversation and is promptly stabbed.
While being stabbed, he denounces the treachery and encourages Fleance to run away and eventually take revenge.
In the meantime, the torch has gone out, and Fleance takes advantage of the darkness to escape.
With Banquo dead and Fleance on the run, the murderers head off to the dinner party to report the half of the job they've done.