So far three sequels have followed the film, Insidious: Chapter 2, Insidious: Chapter 3 and Insidious: The Last Key, along with the possibilty of a potential crossover with the 2012 horror film, Sinister.
At the beginning of the film, a shadowy old woman is seen inside a house while the inhabitants sleep.
Renai and Josh Lambert have recently moved into a new home with their three children. One morning, Renai looks through a family photo album with her son, Dalton, who asks why there are no pictures of Josh when he was a child. Renai reasons that he has simply always been shy (but this is not the real reason; the real reason is revealed halfway through the movie) and disliked taking photos of himself. One evening, Dalton sees the attic door open and goes to investigate when peculiar sounds come out of the door. After entering the attic, he attempts to climb a ladder to turn on the light, but falls when the ladder breaks in the middle. As he falls to the floor, he seems to stare in horror into the darkness as if looking at something terrifying. Shaken, he is put into his bed by Renai and Josh and told not to play in the attic because she believes it is too dangerous for children and she has now made the attic off-limits to Dalton. The next day, Dalton does not awaken from his sleep. Renai and Josh rush him to the hospital, where the doctors say he is in an inexplicable coma and emphasizes that "he has never seen anything like it".
Three months later, Dalton is moved back to his home while still in a coma. Shortly after, disturbing events begin to occur. The first is when Renai hears a rather eerie voice on the baby monitor which shouts "I want it now!", and later when she spots a bloody hand-print on Dalton's bed sheet and a frightening man in her infant daughter's bedroom. This "man" seems very peculiar to Renai because after he attacks her and she shouts to Josh for help (who is sleeping downstairs) the man disappears completely. Renai becomes more disturbed when their youngest son, Foster, says he does not like it when Dalton "walks around" at night. Renai tells Josh about the events, but when she is assaulted once again by the strange man that night, she immediately tells Josh she cannot stand living in the house anymore and they soon move to another house.
In the new house, the supernatural events continue to occur. Renai is cleaning the house when she witnesses a strange boy dressed in old fashioned clothing dancing to some music. It does not stop there; the occurrences become increasingly sinister. Lorraine, Josh's mother, recalls having a strange dream of going inside Dalton's room in the night and seeing something standing in the corner. It looks almost like a human but is not human at all. When she asks "Who are you", it answers "A visitor". Then she asks "What do you want?" to which it replies "Dalton." Subsequently, Lorraine sees a red-faced black figure standing behind Josh that roars at her and Dalton is then violently attacked in his bedroom. This prompts Lorraine to contact a friend, Elise Reiner, who specializes in the investigation of paranormal activity. The family, Elise, and her team enter Dalton's room and Elise is slightly disturbed by something she sees on the ceiling. She provides a visual description of "it" to one of her two assistants who draws a black figure with a red face and dark hollow eyes on the ceiling of Dalton's room; the same figure that Lorraine had seen before in the house; the figure that wanted Dalton.
Elise presumably knows that Dalton had been in an unnaturally long coma and she assumes that Renai and Josh are williing to accept any explanation as to why Dalton is not walking up, no matter what they've tried. Elise explains to Renai and Josh that Dalton has the ability to astral project while he sleeps. (Astral projection, as Elise eplains, is the ability for a person's spirit to leave the physical body when the person is asleep. The spirit can go anywhere but the physical body remains inactive, as though it is "dead".) and that he has been doing it since he was very young. The reason that Dalton is in a comatose state is because he has fearlessly traveled too far into a particular spiritual world (he believes the projections are dreams) and has consequently become lost in a land called "The Further"—a place for the tormented souls of the dead, and, Elise adds, that it is also a place "not meant for the living". While Dalton's spirit is in this other world, he has left nothing but a lifeless body. The tormented souls who are technically "trapped" in the Further crave another chance to live again, and a way to do that is to cross over into the human world and possess Dalton's body through his current state. There are others too (possibly the old woman and the frightening man) who have a more evil agenda and try to use him, and then there is also the red-faced figure, revealed to be a demon, who wants to use Dalton for a more malicious intent, one of which is to "cause pain to others'. However, for a spirit to consume a body, a period of time and energy are required.
At first, Josh gets angry with Elise because of the simply unrealistic and crazy theory she has provided, that Dalton is "nearly dead" because he had gotten lost in some bizarre spirit world where a demon tries to possess him to do bad stuff. However, Josh later relents when he discovers Dalton has been drawing pictures that portray the demonic figure Elise described. Also, Dalton has also labeled one of his drawings "Last night I watched myself sleep, and then I flew away". This simple sentence becomes evidence that Elise was saying the truth. They run a session to try to communicate with Dalton where the demon uses Dalton's body to fight the group, along with other entities who want Dalton's body. After the session, Elise calls Lorraine and the two reveal to the couple that Josh also has the ability to astral project and was terrorized by an evil spirit during his childhood. Lorraine shows them pictures from Josh's childhood, revealing a shadowy old woman (the same woman from the beginning of the film) behind him. Throughout each photo taken of Josh as he grew up, the shadowy woman gets closer and closer to Josh until she is inches away from him, explaining his fear of photos and why there are no photos of him as a child. Elise suggests that Josh should use his ability to find and help return Dalton's soul, to which Josh agrees because there is no other way to bring Dalton back to life.
In preparation of crossing over into the Further and finding his son, Elise sits him in a chair and places him in a trance. Josh suddenly awakes to find that he has astral-projected—seeing his own self asleep in the chair as well as the others in the room. He proceeds outside in a misty emptiness in an attempt to find his way to Dalton. After encountering a boy who points him back towards a house (the same home that the Lamberts moved out of), he proceeds, only to encounter a family who is shot by a bizarre, smiling teenage female of the family in the living room. Startled, Josh makes his way to the attic where he discovers a red door (the same one Dalton drew). Before he can enter, the violent man that assaulted Renai in their daughter's room appears and attacks him. Once defeating him, Josh walks beyond the red door.
Inside is "The Further" and the red-faced demon's lair. While entering a cavernous, red room, Josh discovers a sobbing Dalton, chained to the floor. Josh frees his son, but the demon unfortunately senses Josh's presence and attacks them. Desperate to be reunited with their physical bodies, Josh and Dalton flee the demon's lair, with the hideous demon in pursuit. Just before the two awaken, Josh leaves his son to confront the shadowy old woman who appears to be inside his house. As he shouts for her to get away from him, screaming that he isn't afraid of her, she retreats into the darkness. Moments later, Josh and Dalton both awaken, just as all the spirits vanish.
With the family now happily reunited, Renai, Dalton, and Lorraine chat in the kitchen as Elise and Josh pack up from the long night. Josh hands Elise the pictures from his childhood, and as she takes them from his hands, she senses something awkward as she notice that Josh's hands are old and dirty like an old man and takes a picture of Josh. He then goes into a rage, screaming that she knows that he doesn't like to get photographed, and leaps on her before strangling her to death. Renai hears Josh yelling and goes into the room to find Elise dead and Josh gone. She searches for Josh and finds everyone is gone, the house dead silent. She looks and comes across Elise's camera, seeing a picture in it of the shadowy old woman. It's revealed that what Elise saw was Josh's old and dirty hand and nails, similar to the old woman's, implying that she has possessed him. Josh then puts his hand on Renai's shoulder, saying "Renai, I'm right here," and horror envelops her face as she looks behind her.
In a post-credits scene, the shadowy, old woman can be seen blowing out a candle and the screen fades into total darkness.
- Dalton Lambert - Ty Simpkins
- Josh Lambert - Patrick Wilson
- Renai Lambert - Rose Byrne
- Lorraine Lambert - Barbara Hershey
- Elise Rainier - Lin Shayne
- Tucker - Angus Sampson
- Specs - Leign Whannell
- Foster Lambert - Andrew Astor
- Cali Lambert - Unknown
- Lipstick-face Demon - Joseph Bishara
- Old Woman - Phillip Friedman
- The Long-haired Fiend - J. Larose
- Doll Girl - Kelly Devoto/Corbett Ruck
- The Dancing Boy - Ben Wolfe
Insidious has received generally positive reviews. Review aggregate Rotten Tomatoes reports that 66% of critics have given the film a positive review based on 162 reviews, with an average score of 6.0. The critical consensus is: "Aside from a shaky final act, Insidious is a very scary and very fun haunted house thrill ride." Roger Ebert gave the film 2 1/2 stars out of 4 saying, "It depends on characters, atmosphere, sneaky happenings and mounting dread. This one is not terrifically good, but moviegoers will get what they're expecting." Steve O' Brien from WCBS-FM says " Most Terrifying Film since The Exorcist".
A number of negative reviews reported that the second half of the film did not match the development of the first. Mike Hale of The New York Times wrote that "the strongest analogue for the second half of Insidious is one that the filmmakers probably weren’t trying for: it feels like a less poetic version of an M. Night Shyamalan fairy tale." Similarly, James Berardinelli commented, "[i]f there's a complaint to be made about Insidious, it's that the film's second half is unable to live up to the impossibly high standards set by the first half." Ethan Gilsdorf of The Boston Globe wrote that "[t]he film begins with promise" but "[t]he crazy train of Insidious runs fully off the rails when the filmmakers go logical and some of the strange gets explained away as a double shot of demonic possession and astral projection."
Positive reviews have focused on the filmmakers' ability to build suspense. John Anderson of The Wall Street Journal explains "[w]hat makes a movie scary isn't what jumps out of the closet. It's what might jump out of the closet. The blood, the gore and the noise of so many fright films miss the horrifying point: Movie watchers are far more convinced, instinctively, that what we don't know will most assuredly hurt us... Insidious establishes that these folks can make a film that operates on an entirely different level, sans gore, or obvious gimmicks. And make flesh crawl." Michael Phillips of the Chicago Tribune wrote: "director James Wan and screenwriter Leigh Whannell admire all sorts of fright, from the blatant to the insidiously subtle. This one lies at an effective halfway point between those extremes." Peter Travers of Rolling Stone commented: "Here's a better-than-average spook house movie, mostly because Insidious decides it can haunt an audience without spraying it with blood." Christy Lemire of the Associated Press stated: "Insidious is the kind of movie you could watch with your eyes closed and still feel engrossed by it. It's a haunted-house thriller filled with all the usual creaking doors, groaning floors and things that go bump in the night, but it'll also grab you with some disturbing, raspy whispers on a baby monitor, a few melancholy piano plunkings and the panicky bleating of an alarm as a front door is mysteriously flung open in the middle of the night." When the camera cuts to the scene of a toy clown in a demonic closet, famous singer Michael Jackson's face can be seen in the closet.