The plot of the film is built around the summer solstice day. This ancient holiday is shrouded in a mystical aura in all the cultures. For example, for many centuries the unique rites are held on this day in a Swedish settlement cut off from the civilized world. To explore their traditions, a group of young American anthropology students goes there. However, soon after arriving the friends find out that the local rituals aren't harmless at all. They face such terrible rites as bloodletting, forced insemination, skinning, burning people and so on.
Here we see again how the difficult relationship between close people leads them to a real nightmare. One of the students Christian is tired of being with his girlfriend Dani and is looking for a way to break up. But he has to stay with her out of a sense of duty because Dani sufferes emotional trauma after her sister killed their parents and committed suicide. It is ironic and creepy at the same time that this drama turns into a hell on the summer solstice holiday, which name and atmosphere should cause joy and a smile. Perhaps, that was a subtle idea of the director.
There is a mention of ancient Greek mythology in the film. This is done in order to pay viewers' attention to the similarity of the film's plot with the ancient Greek fatalistic formula of an absolute inevitability. From the outset of the movie there is no possibility for Grahams to change anything. For example, we know that all of Annie's older relatives suffered from mental disorders which led almost all of them to a terrible end. Annie brings this inheritance to her own family and lets the dark forces in their house. (Perhaps that's why the film is called Hereditary. Russian translation "Реинкарнация" is not exactly accurate, though it somehow reflects the storyline.) These forces begin to rule in the house, and any Grahams' attempt to control what is happening is doomed in advance.
This film is an occult horror consisting of metaphors about fear, grief and guilt. These feelings are filigree expressed here in various details - small figures that Annie makes in the basement and manages by herself, pigeon heads that Charlie plays with, mysterious blood inscriptions on the walls of the house, etc. Finally, the guilt and fear permeating the film are the guilt and fear of relatives, both living and dead. The family is presented here as a source of an existential horror. Such an inconvenient and unpleasant topic throws the viewers out of their comfort zone. But that's why Hereditary has such a strong effect.