Will (Logan Marshall-Green) drives with his girlfriend, Kira, through the Hollywood Hills because they got an invitation to attend a dinner party at the home he used to live in with his ex-wife, Eden (Tammy Blanchard). Kira mentions that Eden met her new husband while in grief counseling, before their divorce. Will unexpectedly hits a coyote with his car. To end its suffering, Will kills it with a tire iron.Will and Kira get to the home in the canyons of Los Angeles. Upon entering, Will has a quick flashback of a young boy coloring in the living room. He is greeted by friends he hasn't seen in two years Tommy and his boyfriend, Miguel; a woman named Gina who mentions her boyfriend, Choi, is running late; and former friends, Claire and Ben. Eden and her husband, David, approach Will and he reunites with them, too. David mentions they're going to celebrate that evening. Tommy tells Will to try the wine because it is very expensive. As everyone toasts, Will notices a half-naked woman eyeing him from a bedroom she is Sadie and she soon emerges and is introduced. Eden tells the group that she met Sadie in Mexico. Kira tells the group about how Will hit a coyote with his car and killed him with a tire iron; David admonishes that Will was putting him out of his misery.Will ventures into the kitchen to get a glass of water. He has a flashback to a pain-stricken Eden trying to kill herself with a knife over the same sink. Eden joins him in the kitchen. He asks her why there are bars on the window; she tells him she got it for security from when she was living there alone. She mentions her time in Mexico again and tells him she is different and free from pain now. Ben interrupts and Eden now tells Ben that grief and pain are just chemical reactions and they can be changed and expelled from their bodies. Ben tells her she sounds like a pamphlet and teases her about her new philosophy. She tries to take this in jest but when he tells her the philosophy sounds crazy, she slaps him hard across the face and exits. Ben asks Will how hes been doing, given what hes gone through. Will seems broken and somber still.Eden apologizes to Ben when they reemerge from the kitchen and offers him a drink, which he says he will never refuse. Claire steals Will away and they catch up on things. It is mentioned they haven't seen each other in years. A stranger arrives named Pruitt (John Carroll Lynch) who seems beloved by Sadie, Eden, and David. Gina complains about Choi being late; she complains about not having service up there in the hills. David locks them all inside and notices Will staring at him as he does. He explains to Will that there was a home invasion in the canyon about a month earlier. Will suggests David leaves the key in the door in case they need to get out in an emergency. David complies. Eden asks Will to get some more firewood.Will pauses by a bedroom but then continues on to the backyard to get the wood. He has a flashback of a childrens birthday party in his mind. From outside, Will sees Eden enter her bedroom and shove something into a night stand. Will returns inside with the wood and then sneaks into Edens room. He finds a bottle of pills in an unmarked bottle and pockets a few. He looks at the master bathroom and has a flashback to Eden and him in the bathtub and their young son walking in to say goodnight.In the living room, everyone is talking about how Eden, David, Sadie, and Pruitt are in a cult called The Invitation, which is why they were in Mexico. The others are lighthearted about this, teasing them. David tells them its a group that comes together to help each other in their grief that now has thousands of members all over. As they inquire more, David says he wants to show them a recruitment video. The video is hosted by a leader who tells them that the human brain has the ability to heal and he has made his life work to rewire traumatic experiences; he says they are not alone and they transcend. It then cuts to a woman in bed, surrounded by the leader and dome members as they assist her in suicide. In the living room, this scene unnerves the non-members. Eden tells them the woman was sick and she let her death be filmed to show others that its nothing to be afraid of. David tells them that he used to do cocaine to deal with his grief but he has been centered since joining The Invitation. Eden says that she is finally happy despite the grief she used to feel everyday.The doorbell rings. Pruitt and David answers the door and talks to some unseen neighbors. At the same time, Gina approaches Will and apologizes for keeping her distance over the years. She explains that she had been trying to give him space to deal with his grief. But Will is distracted by David at the front door; David tells Will that it was just some neighbors looking for a party.Will shows the pills he took from Edens bedroom to Miguel, who is a doctor. He is told that its a barbiturate which is only dangerous if you take too much of it. The group is given wine again and they all toast. Will asks Tommy and Miguel if they feel that the night is strange but Tommy tells him its just a friendly, harmless reunion.To apologize for forcing everyone to watch the video, David suggests they all play a game that he learned in The Invitation called I Want where everyone just expresses what they want. Sadie starts, saying she wants to tell everyone that she loves them, naming everyone by name. She then says a lot of people don't say they're afraid but they are and she loves them. Sadie then kisses Gina hard on the lips. Gina then says "I want some of that coke you used to do to David". He actually has some; she tells him she was kidding and he says she wasn't, which was the point of the game. She accepts. Pruitt says, I want to tell you about my wife, Margaret. He tells them that she died when he got in a fight with her and after he assaulted her, she fell and hit her head and died. Pruitt tells them he served jail time for the mistake but it didn't help rid him of the angry person he was. It wasn't until he joined The Invitation that he became a better person and he cant wait until the day Margaret and him can be together again.Claire is uncomfortable, feeling like they're being sold something with all the talk of The Invitation. Sadie interrupts to tell them, "I want to kiss Ben". Ben plays along, giving her a peck but she full-on kisses him instead. Tommy says he wants a blowjob. Claire says she feels uncomfortable and wants to leave. David tries to convince her to stay but Will tells him he should let her leave if she wants to. Claire says goodbye to everyone but Pruitt tells her he parked in front of her and will have to move his car. The two of them walk out. Will watches from the window, suspicious. Sure enough, Pruitt does move his car to let Claire out. Will is appeased at first but then Pruitt stops Claire as she is about to drive away. David interrupts as Will is watching, convincing Will to talk to him alone.David asks why Will is so suspicious of him he says Will seems distant and off, worried about David locking the door or Pruitt moving his car. Will apologizes, saying maybe he wasn't prepared to be at the home he used to live in with Eden and their son.The group has dinner and Will is mentally distracted, flashing back to a birthday party where his son is accidentally killed in some unseen way.Will goes outside in the backyard to clear his head and he tries to get a signal on his phone to find out why Choi isn't there yet. When he returns to the house, he sees Sadie looking in the mirror, making grotesque facial expressions. He goes out in the backyard again and Sadie follows behind. She begins hitting on him, telling him she suspects things aren't going well with Kira and tells him he can fuck her right there. She goes into a speech about not denying themselves of pleasure and that in Mexico, men and women would often have sex spontaneously. She tells Will he can make him like her so much, she can make him beg her without even touching him. He rejects her advances and tells her to go back inside.Tommy greets Will in the backyard. Will voices his concerns about the party but Tommy tells him Claire left because shes sexually awkward and Choi is unreliable so his absence isnt suspicious. Tommy tells Will his weird behavior is freaking people out. They hug and Tommy goes back inside. Wills phone chirps as he is able to get a signal briefly. He listens to his voicemail it is from Choi who says he has arrives at the party early and is about to go inside but forgot the dessert he promised. He asks Will to pick it up instead so he doesnt have to go back down the canyon.Will joins everyone else at the dinner table. The lights are turned out and Eden comes in with a birthday cake for Miguel, even though its a week away. When the lights are turned back on, Will asks where Choi is, telling them about the message he received when Choi was right outside. Eden and David say Choi never got there and everyone tries to assuage Will. But he chastises the others for being polite and continues demanding to know where Choi is. He asks why they were invited to a dinner after a two-year absence, with two strangers from the cult being present. He chastises The Invitation and says its a brainwashing tactic and the cult teaches them just to deny everything by erasing their grief. He begs Eden to tell him where Choi is and asks David why there are bars on the window and the doors are locked. He also points out that he found the large bottle of pills in Edens room. Kira tries to convince Will to leave with her but then the doorbell rings. It is Choi who says he had to turn around after arriving early because he got a call from work. Everyone stares at Will as if hes officially lost his mind. He apologizes, in tears, to Kira for questioning The Invitation.Will goes to the bathroom to wash his face and has a flashback to teaching his son how to wash his hands. He goes out to the backyard and is greeted by Kira. He tells her hes clearly not okay because he is still dealing with the pain of his sons death. He tells Kira he doesn't think she can help which upsets her and she goes back inside.When Will goes inside, Eden is cutting the cake. He asks if he can see their sons bedroom. Will walks around and has a flashback to his son sleeping inside. Through the window, Will sees David go into the backyard and light a red lantern perched on the edge of the backyard. Will retrieves the laptop that the recruitment video was played on. He watches more of the video from the leader who tells the members to not be scared and to remember that a reward is upon them if they take the steps and believe and give themselves over. Pruitt knocks on the door and tells Will everyone is waiting for him, while simultaneously on the video, the leader tells the watching cult member that he loves them.Will is led upstairs where everyone is poured another glass of wine. The members of The Invitation seem elated at this. David suggests they should all take a moment to look at each other and then raise their glass in a toast, to a better world and to peace. Gina drinks her glass right away. Will smashes the glasses out of everyone's hands, telling them not to drink. He tells Kira they're leaving. Sadie goes berserk, telling Will hes ruining everything. She charges at him and he pushes her away, knocking her into a dresser. Everyone surrounds her, worried that she has been killed when hitting her head (akin to Pruitts story about accidentally killing his wife). The group gathers around, trying to revive Sadie. Kira says she's not breathing and it is revealed she doesn't mean Sadie but Gina, who is foaming at the mouth. Miguel tries to revive Gina with CPR but is shot with a gun by David. Pruitt takes the gun away from David while Kira, Ben, and Will back away. Pruitt then shoots Choi dead before venturing to another part of the house. The three non-members try to escape, grabbing Tommy with them, but he is angry about the death of his boyfriend and charges at Eden, who slashes him with a knife.Kira and Will separate from Ben and are charged at by Sadie. Will shoves her away and they barricade themselves in a room but the windows are barred. They can see David in the backyard stabbing Ben and then Pruitt joining, shooting Ben dead. Will tries to calm Kira down, telling them they will survive by doing whatever it takes because they're dealing with human beings. They step back into the house where Will spies David trying to calm down Eden who is upset about all the murder taking place in exchange for the poisoning. David points out that they were chosen and this is how they can leave the Earth along with all the hurt they harbor. Will waits until both have gone and then Kira and him start upstairs to an exit that leads to the garage. Will spies Sadie dying on the couch, having impaled herself with a fireplace poker. As she slowly dies, unable to make noise, he takes the fireplace poker from her lap.Upstairs, Will learns the door to the garage has been removed over the last few years. Pruitt ventures upstairs with a gun. Will emerges, knocking it out of his hand. They scuffle. Kira attacks him with the fireplace poker until she successfully kills Pruitt. They run into Eden in the hall. She shoots Will in the shoulder. Eden then apologizes and shoots herself in the stomach. Kira grabs the gun.Now David is the only cult member alive; he is brandishing the butcher knife, calling for Eden. Will tells David that Eden shot herself and David is glad, because shes with them now. Kira realizes that the gun is empty. Will warns David not to approach because they have a gun but David isnt worried about his own death because its time. Hes stopped short by Tommy who has survived the stabbing. Tommy wrestles David to the ground, stabbing him in the stomach.Nearby, Eden has not yet died. Crying, she tells Will that she also misses their son. She asks Will to take her outside to die. He does and Will, Kira, and Tommy watch as Eden takes her last breaths. The sounds of sirens are heard in the distance, as well as dogs barking, faint screaming, and helicopters churning overhead. As Tommy goes inside to retrieve the corpse of his boyfriend, Kira and Will venture to the side of the hill and notice the hillside filled with light from red lanterns. Theirs was not an isolated incident; thousands of cult members were also carrying out the same plan that night.