Infant Isla turns up one day in a shipwrecked boat, and is adopted and raised by the town's mayor Bobby Bentham (Clayne Crawford) and his wife Grace (Lara Jean Chorostecki), whose pregnancy ended in miscarriage shortly before Isla's discovery. However, it is soon discovered that Isla has healing powers to cure any ailment suffered by those who are near her, including Grace's mother Faye's dementia; swarms of fish also swim to her when she's in the water, thus protecting the town's food security because it guarantees that the local fishermen will always be able to catch fish they had long lost to larger commercial fishing boats from the mainland.
Bobby, Grace and Grace's mother Faye (Frances Fisher) exploit Isla's power by carefully controlling access to her, essentially cementing their own status as the most powerful members of the community, but alienating the town's doctor Beau (Aden Young) as Isla's power has left him unemployed and alcoholic. He spends his nights getting drunk, visiting Isla the following day to cure his hangover.
It is revealed that the island has been inhabited for generations by the same families, and since Isla's arrival they have shut themselves off from the outside world, becoming a Neo-Luddite colony. Bobby is especially concerned about the mainland becoming aware of Isla and her powers, believing that she will be taken from him to be tested on by the government. The village develops a cult around Isla, hailing her as their great benefactor. Every day the family has 'visitations' when a long line of visitors stand outside their house, eager for a few minutes with Isla and they turn “Many Thanks to Isla” into a communal prayer.
One day, Isla and Beau's son Junior are playing on the island and come across a hive of wasps. Isla becomes distracted by how the wasps gather on her hand but Junior gets stung and becomes scared, prompting Isla to close her fists and kill the entire hive. She makes him promise not to tell anyone what happened.
One night one of the village's fishermen, Dillon, asks Bobby to take Isla out fishing earlier than she normally does. The next day a group of kids, including Junior, play a game they have often done in the past where one of the them eats poisonous berries, returning to Isla's house to be healed by her before suffering from the affects. One of the children, Phillip, eats the berries. The kids then rush to Isla's home but as she is not there, Phillip becomes gravely ill. Faye, Grace and Beau rush him to Beau's clinic while one of the other men is sent to fetch Bobby and Isla. Beau attempts to treat Phillip by pumping his stomach and performing chest compressions when he stops breathing but the child dies just as Bobby and Isla enter. Isla attempts to revive Phillip but is unable to, and the boy's parents are left to grieve.
While attempting to comfort Isla about Phillip's death, Bobby and Grace notice that Isla's healing powers have seemingly disappeared when Grace cuts herself while preparing dinner and it does not heal. Bobby then intentionally cuts his hand and notices that it too does not heal. They call a town meeting to discuss Isla's lack of powers and instruct Beau to re-open his clinic. This causes a rift amongst the villagers. Bobby wishes to cease Isla's visitations and potentially get her help from the mainland. The majority of the other villagers however resist this idea and Faye calls a vote on whether the visitations should cease or not, with the majority voting to continue the visitations, under the pretense of spending time with Isla to show their gratitude and love.
One night, it is revealed that a group of villagers regularly hold a support group, led by Faye, who is becoming the de facto leader of the island, despite her dementia slowly returning. Grace shares with Bobby her fear of Faye's impending decline if Isla cannot heal her.
One night, Grace notices that Isla's healing powers have returned but only when she is asleep, and she shows this to Bobby. Bobby tells her that they must keep this a secret. After Faye has a bout of dementia, Grace, in a moment of weakness, takes her to see Isla at night, and Faye too realizes afterwards that Isla has her powers, but only when asleep. Dillon returns to Bobby the next day and tells him that the fishing yields have been too low, especially after having to give up their entire catch the day of Phillip's death, implying that they need Isla to help them. Faye later speaks privately with Bobby, giving him her stash of Temazepam from before they closed off the island, to drug Isla so that they can take her out on the water and fish, in order to survive the coming winter.
Meanwhile, Phillip's parents, Frank and Nancy, have become disillusioned with life on the island and secretly build their own boat from parts of their house in order to leave the island with their surviving daughter, Susan. Beau discovers this, and he shares with them that he too wishes to leave the island with Junior. Frank tells Beau that they will leave in a few days, and they have space for two more people on their boat.
The day after taking a drugged Isla out fishing, the villagers gather to process the fish and eat together. Beau talks with Bobby and Bobby confides to him about drugging Isla, and that he wishes to take her off the island. Beau shares with him Frank's plan, and tells Bobby that they can take Isla and him, but not Grace. Faye witnesses this conversation from afar, and becomes suspicious.
That evening, Beau returns home to find Faye in his kitchen. They have a thinly-veiled conversation about each other's motives, and Beau feigns loyalty to Faye and the village, telling her that he will stay with her as she succumbs to dementia, which offends her. Faye leaves as Beau drinks from a whisky bottle, but falls over and sees that Faye has laced the whisky with the same poisonous berries that killed Phillip. His house is soon mysteriously set ablaze while the villagers watch helplessly. Frank and Nancy use the distraction of the fire to flee the island. Bobby takes Isla to meet them but discovers that he is too late and that they have already left.
The next day, another one of the island's inhabitants, Marlon, sounds an alarm on the island as he spots a police boat approaching the island. Dillon fetches a revolver that he had stashed, and he and Bobby rush to greet the boat at the marina. The police officer introduces herself and a Social Services agent, Emily, and explain that after Frank and Nancy reached the mainland they expressed concerns about Isla and they have come to check about her welfare.
Bobby takes them to his house, while Dillon defiantly observes, and the entire village gathers at the house. Emily goes into the house to speak with Isla while outside, tensions arise as the police officer notices Dillon becoming confrontational and asks him to back off. Separately, Junior inspects the police boat, finding a shotgun. He then uses the radio to ask for more people to come, and runs off towards Isla's house.
Inside, Isla assures Emily that she is fine, despite confirming that her parents sometimes argue. As Emily moves to exit Isla's bedroom, Isla tells her that she is supposed to say "Many thanks to Isla" as she leaves, which alerts Emily. Emily returns outside as Dillon and the police officer continue their confrontation. Other villagers begin to surround Emily and the police officer, while Bobby futilely attempts to defuse the situation. Dillon and the police officer both draw their guns on each other, while two villagers grab and restrain Emily. The scuffle startles the police officer who accidentally shoots Bobby in the chest, and Dillon shoots and kills her.
Faye, Dillon and other villagers rush Bobby inside. Bobby desperately asks Faye to fetch Isla but Faye forces him to swear fealty to her before she will get help. He agrees, and Faye goes upstairs with a glass of water spiked with more Temazepam. Upstairs in Isla' bedroom, Grace and Faye restrain Isla and attempt to force her to drink the water. Junior bursts into the room, brandishing the shotgun from the police boat and telling them to leave her alone. As Faye moves to snatch the gun from him, Junior shoots Faye. Isla screams and closes her fists, just as she did when Junior was attacked by the wasps.
Isla opens her eyes and slowly walks out of house, revealing that she has killed Junior, her mother, everyone in the house, and everyone on the island. She walks slowly back to the cliffside she used to go to with Junior, and watches as multiple police boats arrive at the island.
