Ring didn't really need a sequel but since it was a massively popular horror movie, it got one anyway (and several others after this). This one picks up immediately after the conclusion of events in the first movie and has characters investigating further the mysteries of Sadako. Aside from being unnecessary, I guess one of the main issues is that the more you explain about Sadako, the less scary she becomes - the unknown element works in the first movie's favour, allowing us to fill in the gaps. Nevertheless, this is still quite decent, with some very effective sequences; chief amongst them being a scene where the protagonist has a dream that replays events from the weird videotape showing Sadako and her mother by the moving mirror. It's a super creepy sequence and along with several other nicely sinister moments, ensures that Ring 2 is worth watching.
Review of Ring 2
