The subterranean race known as the Lava Men (Thor fought one of them in his own book several months ago) have a problem. A big chunk of unstable and explosive "living rock" is growing rapidly inside their kingdom. If it goes off, it'll blow up the planet.
Their solution? Push the rock to the surface, where it'll only take out all us annoying surface dwellers. The rock happens to appear in the American Southwest, near where Bruce Banner has recently returned to work for the Army.
So he's nearby when the Avengers show up to investigate. The bulk of this issue is a battle between the Avengers and the Lava Men, with the Hulk appearing near the end. The Avengers manage to trick the Hulk into helping to harmlessly destroy the living rock.
It's a pretty good issue with Kirby's usual well-choreographed action sequences. There's a couple of other notable points:
a) The various Marvel comics continue to build a strong internal continuity. This one, for instance, picks up right after the events in Fantastic Four #26, with the Avengers taking stock of damage to their headquarters done by the Hulk. And it's a way of building continuity that is perfectly fair to the reader--if you haven't read FF #26, you haven't missed anything you need to know to follow this story.
b) This is the Hulk's last regular appearance in the Avengers. The big green guy will pop up in Spider Man in a couple of months and finally get his own series again as part of Tales to Astonish in four months--the start of a long-running series that will mark him as one of Marvel's most important characters.