Insidious: The Last Key is the fourth film in the Insidious horror series and the second prequel after 2015's Insidious: Chapter 3. The filmmakers are wise to center the prequels around the most interesting character in the series, parapsychologist Dr. Elise Rainier played by wonderful character actress Lin Shaye. You will remember Shaye as the landlady in Kingpin (1996) or as the overly suntanned Magda in There's Something About Mary (1998).
In The Last Key, Rainier is on a personal quest, as the haunting she's investigating is very familiar to her because it is taking place in the house she grew up in. So, she's literally haunted by the demons of her past. Of course, backing her up in this investigation are her two younger, bumbling sidekicks – Specs (writer of the Insidious films Leigh Whannell) and Tucker (Angus Sampson).
Some of the dialogue here is simplistic and cornball, and it is kind of creepy when Specs and Tucker are hitting on two younger sisters (Caitlin Gerard and Spencer Locke, whose characters because important to the plot), but The Last Key has a lot going for it. Surely it wouldn't be a horror movie without some silly jump scares, and The Last Key does have a couple of decent jolts. There's also a wicked looking demon that has skeleton keys for fingers, but what makes the movie work is its emotional center which is perfectly realized by Shaye.