Evil Dead II is a remake/continuation of the first Evil Dead. The first twenty minutes or so summarize the first movie, and then the rest of the movie takes place directly after the events of Evil Dead.
Ash and his girlfriend Linda drive out to a deserted cabin in the woods for some peace and relaxation. Meanwhile, the daughter of the couple that owns the (now abandoned) cabin is on her way, meaning to help her father translate the Book of the Dead (a common enough family activity back then). Her boyfriend and a hillbilly couple are along for the help. But before that can happen, Ash plays a recording of the father speaking aloud verses from the book and suddenly there are demons all over the place!
The effects in this film are pretty amazing, from the fantastic introduction with a great stop-motion effect of the Book of the Dead writing itself overlayed across an image of a roaring sea of blood to the final battle with a giant demon head in the woods.
Also amazing is Bruce Campbell as Ash. It's almost like it's a different person playing Ash this time around! Every scene without Ash suffers in comparison.
However, although Evil Dead II is a much better film when it comes to effects and acting, it's not really scary at all. On top of that, the movie has pacing issues. Once the other characters arrive at the shack, the movie kind of drags and there are a couple of scenes where Ash is possessed and it just really doesn't work well. And there are too many scenes of people creeping over to doors to check out some noise. In a straight horror movie those scenes work to build suspense, but since Evil Dead II is more of a slapstick gore comedy they feel out of place and just end up killing the momentum.
Regardless of its status as a horror movie, Evil Dead II is a fantastic way to spend ninety minutes, assuming you don't mind a little bit of red karo syrup with your slapstick.