Hereditary opens with a terse obituary. An old woman has died, leaving behind her adult daughter Annie (Toni Collette), Annie's husband Steve (Gabriel Byrne), and the couple's two teenaged children.

Written and directed by first-time feature filmmaker Ari Aster, this horror film grabs your attention with an absorbing opening act that shows how the Graham family is handling the death. Annie, who works meticulously on gallery-exhibited miniature dioramas of real traumatic experiences from her life, reluctantly attends a support group and opens up about the strain that marked her relationship with her mother. Steve keeps his cool, shielding Annie from a troubling development relayed by the funeral home. Meanwhile, the kids aren't really grieving as much as they are trying to endure their awkward years. Peter (Alex Wolff) quietly crushes on a classmate as his mind wanders and the eccentric Charlie (Broadway-seasoned Milly Shapiro) draws pictures and does weird things that only a horror movie kid would (like severing the head of a dead bird outside her school).

At its start, Aster's film is methodical yet arresting, its original, offbeat presentation subverting expectations with a few different kinds of creative scares. Then it completely disarms with a shocking scene you can't expect. By this point, you're convinced of two things: anything can happen in this movie and this is one of the good horror movies that isn't just recycling old tropes or reviving old triumphs.

Unfortunately, it's all downhill from there. The rest of the extremely excessive over two-hour runtime slowly attempts to build tension, seemingly by setting up some big twist that breathe coherence into all the inexplicable weirdness that is unfolding. But that big twist never comes in any discernible form. The weirdness remains inexplicable throughout. Jarring sometimes and even slightly chilling, but nothing ever comes close to adding up in any kind of satisfactory way.

At her support group, Annie meets Joan (Ann Dowd), an older woman who's lost a child and a grandchild. Joan tips Annie off to the value of open séances and soon this slow burn that began steeped in dark, uncomfortable realism ventures out into the realm of the supernatural. Hereditary, whose title is among the many things that never quite add up, betrays the good will it develops over the course of a solid hour or so. It intrigues with the occasional sequence only to render them fantasy. That repeated practice leads you to doubt everything that follows, whether it's having to burn a diary in a fire or a bit of self-inflicted harm.

Collette is a talented actress, and she is joined by agreeable seasoned vets like Byrne and Dowd, but the respectability they bring to their roles gets drowned out by the random, laborious final act. It's a shame that a film that does so much right in its early goings can leave you so bewildered and disappointed by a second half that isn't compelling enough to even try to decode. Not since mother! has a movie so gloriously failed to deliver on early promise that it cultivated.