In 1989 on Cuttyhunk Island, scientist Mason Lane used microscopic parasites to advance human evolution. The experiment resulted in stronger, smarter, healthier humans, but something went wrong and three quarters of the island’s residents died.
20 years later, a man pursued by police through the woods. As he reaches the shoreline, the writhing of a tiny parasite is seen beneath his skin. Trapped, the man dives into the ocean. As the police arrive at the shore, they hear a scream and see the man’s remains disintegrating in the ocean.
Teenagers Jamie Ackerman (Mircea Monroe), (daughter of the scientist who spearheaded the project), Kristen, Marco, and Justin, take a boat ride to Kuttyhunk Island. Their lodge is shown to be occupied with another couple who wake up to find their daughter infected with parasites only to be consumed by them as well. The group reaches the lodge afterwards with no idea what happened. Kristen decides to run in the morning before encountering a procession of men and women in mourning. She heads back to her lodge under watch by a mysterious figure.
The film cuts to two scientists, one being Dr. Macavire, studying a cadaver that has previously been infected by the parasite, and the male proceeds to explain that the idea came about from medical history. Parasites like tapeworms or leeches were used to control blood or prevent obesity in earlier time periods, but it was only until recently that they were utilized to alter DNA of their hosts. So the male researcher began genetically engineering parasites to help oysters form the most pristine pearls, and once that project was successful, he decided to develop helpful parasites for humans. For a long time, the project saw fruitful results, but then realized that the humans started suffering since the parasites had to feed. He mentions that every parasite’s weakness has been salt and shows by putting one of the human parasites in a solution of salt water. The research is called out by his superiors, one of them being Larkin and one of the parasites in the cadaver exits the corpse and makes its way to the female scientist. After a verbal scuffle with Larkin and his police compatriot, the male scientist returns to see the woman convulsing, causing the policeman to shoot her in the head.
Kristen returns to Justin who is woodchopping and tries it out for herself. Jamie has more memories of her lodge, remembering her father bringing up the idea to his wife who has some skepticism about the project. Her memories are interrupted by a knock on the door who turns out to be Larkin, and after a minor argument about title deeds, realizes that she will be losing money from her inheritance of Kittyhunk. The film cuts back to the mysterious figure watching Kristen and Justin play a game about truth and lies. Kristen vocalizes her interest in Justin, who at first lies about reciprocating interest before Kristen realizes his ploy. They reach a river and Kristen says if Justin treads water and sees a bird, then he’ll “get whatever he wants.” As he heads out to the river, the mysterious figure is at the other end of the bank, and sends one of the parasites after him.
Meanwhile, Jamie and Marco find a compartment underneath their mattress and proceed to open it. The film returns to Kristen and Justin, and Justin has already decided to exit the river since it is too cold for him. Right as he puts his hands on the bank, Kristen tells him to look at a bird in the sky, so now they wonder what will happen next. Kristen gives him her jacket to warm him back up, and they make-out. They continue the foreplay until the parasite enters Justin’s heel, and immediately he stops and feels his senses heightening.
The film returns to Jamie and Marco, and she has a brief flashback as she touches the clown doll she found. Then they watch a VHS clip of Larkin commemorating Mason Lane the scientist, who begins a speech about evolution and progress. There is a thump behind the window to his left, and when Larkin lifts up the curtain, screams from within are heard and part of the glass breaks. Kristen runs back to Jamie and Marco and tells them something is wrong with Justin. All who attended are evacuated as the VHS changes to another scientist who says the parasites feed on adrenaline and grow within due to their insatiable appetite. The trio return to Justin who is convulsing, and the parasite is seen traveling throughout his body before they pick him up and take him back to his room. Kristen asks specifically about what she saw in his belly, and Marco hastily suggests that it might have just been a muscle spasm. Jamie gets help from Sarah, and they head to the nearest house to borrow a phone to call a hospital, while Marco takes a bike to the hospital. Jamie wonders if they are being followed, but still exits the car to knock on the door. She spots the inhabitant inside, but does not realize that the parasites have already consumed him.
Kristen is now playing an accordion while someone outside is watching her. She heads to the bathroom to pretty herself up and the door is broken open to reveal Justin back to full health. They kiss and Justin says, “Round two?” and Kristen says she’ll take a bath as well. Jamie and Sarah reach another house, but Jaimie again notices the same car parked not too far away, and before she exits the car, Kristen tells them over walkie-talkie that Justin is all better. So they head back; meanwhile, Marco can’t reach anyone over walkie-talkie, and spots a payphone near him. Justin wakes up in the middle of the night, feeling that the ticking of alarm clock is too loud for him. He puts it in a drawer, and walks outside, and feels his eyesight focusing and going out of focus. He hears sounds better now too, as exemplified by hearing a couple having coitus in a cabin or hearing a dog bark far away. He meanders about the woods a bit, which lets one of his parasites detach from one of his calf muscles. He heads back to his cabin and puts in tissues in his ears so the noises don’t bother him as much. He puts his hand down Kristen’s pants, but she says she is too sleepy. Meanwhile, his minion parasite heads up to the cabin of Sarah. Justin puts on clothes and Kristen’s sunglasses before heading to a local bar to get some lady attention. There within, he drinks a bit of whiskey before spotting a target among a group of three. After drinking he picks her up before another man steps to try and break them up. Justin offers to fight outside, and they do. Justin’s heightened abilities enable him to evade and redirect the man and his entourage’s blows, and Justin threatens to throw a chunk of the stone used to make a street before his lady friend stops him from doing so. They make-out and grope before the girl realizes that the parasite is within Justin’s ear, and Justin begins to choke and rip out a part of her neck. Justin wakes up, and wonders whether it was a dream or not. Jamie comes in with breakfast and they discuss a bit about the wedding of their stepparents. Justin suddenly vomits into the toilet and sees numerous parasites in his vomit.
Kristen has gone for another morning run along the same path she took, but breaks off to find a curious trail. Justin has suddenly become good at woodchopping, and breaks wood in half with his bare hands. He punches a nearby tree and looks down at his hand. Kristen has found a large cemetery of numerous individuals, all dated 1989, and the mysterious figure attacks her, only to be thwarted by her mace. Justin finds a parasite writhing about outside the back of his hand, pulls it out, and returns to woodchopping before encountering Marco who tells him he is planning to ask Jamie’s hand in marriage. Justin hears screams twice, and runs off in the direction of Kristen. Kristen believes she escaped the man, but the man catches her from behind and shoves the parasite in his mouth down hers. Marco sits with Jamie, who tells him the financial situation. At that point Larkin and Dr. Macavire ask to see Justin since they traced Kristen’s sunglasses to him by the body of the woman he murdered last night. Jamie and Marco run away to warn Justin, and infected Sarah looks out to see what has been going on. Justin has found Kristen’s mutilated body as the militia approach him. Justin’s pupils disappear and adopt a singular gray hue as the parasites heighten his body’s abilities. He does quick work of every militia, but not before they gun down Marco. Jamie was kidnapped by the mysterious figure before she could reach Justin, and Justin returns to his senses as he realizes the manslaughter he committed. He apologizes to Kristen before running away. Dr. Macavire, a policeman, and Larkin appear to murder any infected individuals and kill the larvae with salt. The policeman notices a parasite sliding back up Dr. Macavire’s leg and shoots him promptly.
Jamie finds herself trapped in the mysterious figure’s house, who reveals himself to be Mason Lane, Jamie’s father, she has another flashback of him killing his first wife, Jamie’s biological mother, in order to obtain the antidote against his parasite. Mason explains he developed another formula to keep the parasites he houses at bay in order to retain their ability-enhancing features. Justin is shown to be even further consumed by the parasites, and begins to run. Jamie manages to escape with an antidote to combat him and alert Larkin. Larkin appears to confront Mason, giving Jamie time to hit Mason with a part of the antidote and escape. Larkin takes the rest of the formula to hurt Mason, but Mason easily parries his strike and breaks the needle. Jamie douses Mason in salt water, causing him to disintegrate and release the bevy of parasites within him. Together, Larkin and Jamie escape the laboratory as the parasites swarm after them.
Larkin reveals that Dr. Macavire was trying to lure Jamie back to the island with suggestions of inheritance of the island, assuming that Jamie had carried the antidote with her. Larkin revealed that his ploy about the loss of money was an attempt to get her back off the island. He also reveals that he managed to get Jaimie off the island because that would enable the antidote to not be destroyed by Mason, and that is why Jamie’s mom died protecting her. They come upon Sarah’s car with Sarah’s head on the horn while her baby is crying. As Jamie tries to take the baby, Sarah reanimates and shoots a parasite at Jamie’s hand, which Jamie promptly removes. Larkin succeeds in strangling Sarah and they return to the car.
Larkin drives her to a boat shack that houses the pearls and salt to deter any parasites. Larkin tells Jamie to get one of the boats to escape, but at that point Justin emerges to stop them. Jamie starts getting the boat to leave with the baby, and Larkin tries to confront Justin with salt. He scalds Justin’s right side of his face, before Justin kills him. He starts chasing the boat, but does not notice that Jamie took a rowboat. Just when Jamie believes she has escaped, Justin appears behind her on the shoreline, reactivates his human side, and pleads with her not to leave him on the island. Jamie still escapes without him, and spots a boat far away. She sneezes, and sees larvae in her snot, and so she dunks her hand in the salt water, only to realize her hand is also reactive to the salt and is therefore infected as well. She realizes that she can leave her clown doll, which carried her personal information, with the baby, and jumps into the sea to destroy the parasites. The final clip of the boat is a few days later when a motorboat comes upon the baby and the boat.
6 months later in Seoul, South Korea, a woman enters a jewelry store to try and return a ring she got from her husband. The requirement of a receipt incites anger in her, and she boisterously announces she will return with her husband. Meanwhile, in one of the glass cases, a pearl ring breaks open to reveal a parasite.