Picking up exactly where the original left off (actually a few minutes earlier; it replays the final scene, though inexplicably changes the music and adds in some different shots), Michael Myers is still on the loose, Dr. Loomis is in hot pursuit, and Laurie is rushed to the hospital. What ensues is essentially rehash, albeit amped up in the violence and nudity department to compete with the wave of slasher movies that was drowning America at the time. There is also a fairly awful plot twist where it turns out that Laurie is Michael's sister, I guess inserted as a pretext for why he'd still be trying to kill her.
Halloween II's biggest sin is, well, being the sequel to Halloween. The original is a masterpiece, one of the best horror films ever made, so how could a silly, crass, generic slasher film hold up? The legion of other silly, crass, generic slasher films from the early 80's aren't held to such a high standard, they are only judged against the Friday the 13th watermark. And you know what? Going by that criteria, Halloween II holds up pretty well as a respectable entry in the subgenre. It's mostly entertaining, looks nice (same cinematographer, Dean Cundey, as the original), has some half-memorable set pieces (apparently John Carpenter did some uncredited reshoots), and a lot of the one-uping outbursts of graphic violence that this species of horror film is known for.
Yet it's hard not to compare it to the original at least a little. First off, the whole idea of sequel kind of ruins the brilliant, ambiguous ending of the original. The genius of the original ending was that it leaves you hanging, the fear never subsides because you don't know what happens next. Only now we know what happens next.
Like many cash-in sequels, Halloween II tries copying several memorable elements from the original with diminished effect. We get the extended POV shot opening, Dr. Loomis's weird ramblings about evil, Laurie locking herself in a confined space... none as well executed as in part one. Most glaring is probably the music; they took the theme from the original and reworked it, spiced it up with a fuller, more synthetic feel, and it loses its minimalist magic in the process.
My favorite tell-tale sequel sign in the film is what they do with Jamie Lee Curtis. She gets the Steve Guttenberg Police Academy sequel treatment. She's barely in the movie for the first hour and change (and spends most of her screen time unconscious or in bed), only to have the movie pretend she's the lead role during the climax. My guess is that she was only willing to work on the movie for a few days.
But look at me, doing exactly what I said I wouldn't. On its own terms, Halloween II works. The mood is nice, the kills are fun, Donald Pleasence is always a joy to watch. At one point a dude slips in a puddle of blood and knocks himself out, that was pretty funny. He has a concussion so later on he passes out in the front seat of his car with his head on the horn, alerting Michael to his location. That and several other clever moments help Halloween II rise above mediocrity.
One last note - this movie has one of the worst, funniest tag lines of all time: "More of the Night He Came Home."