There exist three types of "sequels" in the horror movie industry
Sequels in name only, which have absolutely nothing to do with the events of the original. Normal or regular sequels, which refer or build onward on the events of the original but can be enjoyed separately still. And then finally there are sequels, although a minority, which you simply cannot begin to watch without having seen the original because the story lines of both are interwoven. "Insidious 2" is such a sequel. Don't you just love it when they are making a sequel in order to clarify and explain the convoluted and twisted plot of the original, only to end up with two utterly complex and confusing movies instead of one! Preferably you should even watch "chapter I" and "chapter II" back-to-back to fully comprehend everything. I watched them with an interlude of a few years and really had to dig deep in my memories to remember the details of the first film. But hey, let's first state clear that "Insidious 2" is an admirable fright-and-jump movie from the talented duo Leigh Wannell and James Wan who already proved that they are masters of horror and easily capable to make the hairs in the back of your neck rise. The Lambert family is still being haunted by ghosts regardless of where they live, and they come to the surface via the spiritual mediums Josh and his oldest son Dalton. Elise, the medium that that helped them to save the life of Dalton in the first film died in the last seance, but it seems as if she's still trying to warn the Lamberts for dangers from the other side, and thus Josh's mother Lorraine seeks the help of her old friend Carl. Meanwhile the police are investigating Elise's death and, since they obviously don't believe in ghosts, they want to frame Josh for murder. The main trumps of "Insidious 2" are strong performances (Rose Byrne, Patrick Wilson, Lin Shaye and Barbara Hershey always give their finest performances when directed by James Wan), a very patient but extremely atmospheric and ominous building of suspense, genuinely petrifying shock moments that manage to grab you by the throat even though you can see them coming and a handful of intriguing sub plots. The parallel plot of Parker Crane and his petrifying shrew of a mother, for example, is truly fascinating and re-establishes once again that Wan and Whannell are creative writers.
Review of Insidious: Chapter 2
