This horror film from the series is a prequel set before the Lambert family is haunted as seen in the earlier films.
In Insidious: Chapter 3, we see how Elise uses her ability to contact the dead in order to help Quinn who has been targeted by a dangerous supernatural entity. While connecting with Quinn's dead mother, she comes across demons and powerful souls who are harmful and extremely scary.
The prequel is about Quinn who sees her recently dead mother, Lillith's spirit often and thinks that she has been trying to reach out to her. While reading during a visit to Elise, Quinn is warned by the psychic, "If you call out to one of the dead, all of them can hear you." But by the time they can think and decide, things go out of control as demons are here and trying to get a hold of Quinn's soul.