Clive Nicoli (Adrien Brody) and Elsa Kast (Sarah Polley) are two brilliant scientists who are developing gene-splicing technology to create new species. They have just put the finishing touches on their newest creature, “Fred”, a large dog-sized wormlike creature that produces proteins beneficial in growing crops. Clive and Elsa bring Fred to meet his companion, Ginger, a female version of this new species. The two instantly get along well and start imprinting (weird tendrils come out and start circling one another). Elsa and Clive are pleased that the two are getting along so well.
Barlow (David Hewlett) is excited about the development of the proteins and takes the pair to meet with the head of Newstead Inc, Joan Chorot (Simona Maicanescu). Joan tells the pair that their splicing laboratory will be converted into a protein observation lab so that they can identify and synthesize the gene developing the growth protein in Fred and Ginger. Elsa and Clive immediately object; they had planned to incorporate human DNA into the splicing procedure in order to start work on curing numerous genetic disorders. Barlow makes it clear that the company doesn’t have a choice: They need profits to offset the cost of the splicing technology and the protein is the only way to do that. Joan tells them that in five years, they should be able to start testing with human genes.
Elsa and Clive go back to the lab and immediately start testing to see if they can produce a viable hybrid with human DNA. They try numerous iterations, but get frustrated with each failed attempt. Clive gets lucky and accidentally finds the best fit when Elsa gives him a new genetic sample from a Jane Doe. They produce some genetic material and put it on ice, so that they will have a starting point in 5 years time. On the way to storage, Elsa hijacks the material and takes it to the fertilization portion of the lab, changing the password to the room so that Clive won’t be able to interfere. He reminds her that they could go to jail for what she’s doing but Elsa insists that they won’t carry the fetus to term, just see if it can sustain growth. Clive debates aborting the fertilization, but decides against it at the last minute, allowing Elsa to have her experiment.
Clive and Elsa relegate most of their work onto their team, headed up by Clive’s brother, Gavin. Instead of synthesizing the protein, they spend their time studying the splice fetus as it develops at an astonishing rate. They go home to their apartment after work. Elsa shows Clive an apartment she wants to get, but Clive says he doesn’t feel like moving around after, since it’s not big enough for a child. Elsa says she doesn’t want to accommodate a third party who doesn’t even exist. They curl up and prepare to go to bed, but Clive’s phone goes off: The fetus in the lab is past maturity and trying to leave the artificial womb.
The couple arrives at the laboratory and sees the machinery leaking. Elsa puts her hand inside of the artificial womb in order to pull the specimen out but the specimen bites her and doesn’t let go of her hand. Clive breaks the container and cuts the artificial womb with a surgical scalpel. He stabs the specimen and it falls onto the floor. Clive puts a plastic container over it to keep it from moving and goes to treat Elsa’s wounds.
After bandaging Elsa’s arm, the two go to examine the specimen but it doesn’t move. Clive and Elsa think that it’s dead, so Clive leaves the room to get a scalpel to perform an autopsy. Elsa examines the specimen’s corpse, but discovers that it is merely a shell: The specimen has evolved into something new and escaped the containment box. Elsa sees the weird new creature walking around like a chicken, but crashing into everything. Clive wants to gas the room and kill it, but Elsa removes her mask so that the specimen can live. Over the next few days, the pair examines the creature. They take X-rays of its body and study its habits to determine what it eats and how it develops. It goes from having no arms and a thin torso to developing into a child like humanoid stage, with 4 fingers, weird legs and a tail that has a sharp blade shoot out of it.
Clive is still reluctant to keep it alive, but Elsa continues treating it like a child, dressing it up like a little girl and teaching it. After running several tests, Elsa is able to determine that the specimen has higher level thinking abilities. Elsa calls the specimen “Dren”, as a reversal of the first word the specimen was able to make an association with (nerd, which was written on Elsa’s shirt). Clive tells Elsa that they shouldn’t be treating it as if it were human, when Gavin breaks into the lab. He overhears their argument before Dren attacks him and tries to pierce him with her tail blade. Elsa scolds Dren, which prevents her from killing Gavin. Gavin freaks out and leaves the room. Clive tells Elisa that they need to end this, and tries to go after Gavin.
That night, they put Dren to bed and lay on the couch outside her room. Clive notes that it’s been a while since they’ve had sex and Elsa says that this must be what it’s like to be parents. She gets mischievous and seduces him on the couch. He points out that they don’t have a condom but she says, “What’s the worst that can happen?” Dren hears the noises and observes her “parents” having sex on the couch. Clive sees this but doesn’t say anything. Once it’s over he has a worried look on his face when he sees Dren isn’t there.
Clive and Gavin have coffee in the break room and Gavin rages against Clive and Elsa’s actions. He asks if Clive thought of anyone but Elsa when he made his decision. If Clive and Elsa are caught, then the entire facility gets shut down, everyone loses their jobs and Clive and Elsa go to jail. Gavin tells Clive to learn how to say no to Elsa and leaves.
After going over Dren’s data, Clive points out that there are weird growths, which look like lungs in her chest cavity. Since her lungs are accounted for, they don’t know what it is. When Dren starts getting sick, Clive suggests putting her in a cold bath to keep her temperature down. Dren starts convulsing and Clive sees his opportunity to fix his mistake. He tries to drown Dren and forces her to stay underwater, hoping that she’ll drown. Unfortunately, the growths turn out to be water-based lungs, which allow her to breathe underwater and necessity her staying near water environments. Elsa is happy that Clive figured it out and asks if he knew the whole time he kept Dren underwater. Clive lies and says yes.
A few days later at the stockholders, Clive, Barlow and Elsa present Fred and Ginger. Elsa lowers the separators in their tank in order to replicate the imprinting process from earlier, but instead the two specimens become incredibly hostile. Stingers like Dren’s pop out and the two attack each other, ripping each other to shreds (horrifying the stockholders and Clive and Elsa). The tank overturns, shatters, and sprays the audience with blood.
After the incident, Joan and Barlow make it clear that there will be no more splicing in the lab. Either they isolate the protein or they will not be welcome in the lab. Clive and Elsa go over the data and realize the Ginger morphed from a female to a male, creating a fundamental territory standoff. Clive scolds Gavin for not noticing, but Gavin points out that they’ve been too busy with their monster to do their actual jobs.
With Barlow making changes to the lab, Clive and Elsa move Dren to Elsa’s mother’s old farm. Dren doesn’t like the new barn at first and runs away. The pair goes after her and finds her eating rabbit raw in the woods. When Dren doesn’t eat her food later, Clive suggests that maybe she’s acquired a taste for meat. Elsa insists that Dren is a vegetarian. Clive suggests relabeling rabbit as a vegetable.
While moving into Elsa’s house, Clive notes that Elsa never talks about her time at her mother’s house and after arriving there it becomes clear why: Elsa was treated like a dog by her dominating mother. Her room consisted of a mattress in the corner, a chair and a pot. Clive is horrified at the conditions in which Elsa was raised and starts to wonder just how emotionally damaged she is. While the pair are away, Dren finds a cat in the barn and keeps it as a pet.
Elsa spends more and more time with Dren while Clive goes to work to synthesize the compound. Unfortunately, Fred and Ginger’s corpses have been dead too long and he can’t synthesize anything. Barlow gets pissed that Elsa hasn’t been showing up for work because she’s “sick” but doesn’t do anything about it except bitch.
When Clive gets home, he and Elsa pay Dren a visit in the barn but find that Dren wants to go outside. When they say no, Dren breaks through a window and climbs onto the roof. The pair pursues her to the edge, where Dren falls off. When she falls, her hormones stimulate the release of her wings, and her arms and back shoot out special retractable wings, allowing her to fly. Elsa demands that Dren go back into the barn and Dren complies.
Elsa tells Dren that she loves her and that there is a little of Elsa inside of her. She gives Dren her old Barbie doll and applies make up. She then finds a stack of drawings of Clive, suggesting that Dren has developed an infatuation with him. This upsets Elsa a bit, since there are no drawings of her, and when she sees the cat, she takes it away telling Dren that growing up means that she can’t always get what she wants. Dren gets upset and throws a tantrum while Elsa goes and sleeps in her old room.
Clive arrives later and finds Dren upset. He tries to teach her to dance in order to lift her spirits but notices something weird and familiar. He’s strangely attracted to her and realizes that Elsa used her own DNA to make Dren. He leaves Dren, confused and goes to the house. When Elsa wakes up the next morning, he yells at her for what she did and says that she must be even more fucked up than he thought. He tells her that Dren is her sick attempt at having a child she can control since Dren isn’t human but an experiment. He leaves her and goes to work, still disgusted with what he’s found out.
Dren becomes more and more restless within the barn and craves to go outside. Elsa is surprised by Clive’s outburst but goes to Dren and says that she has a present. She gives Dren her cat back but instead of Dren being pleased, she kills the cat with her spiked tale. Elsa is shocked and slaps Dren across the face. Dren jumps on her and puts the tail blade to Elsa’s throat…cutting off the key to the barn, which Elsa wears as a necklace. Dren tries to make a run for it, but as she opens the door, Elsa hits Dren over the head with a shovel.
Elsa loses her mind and begins studying Dren again, something she had stopped doing while raising her. She says that the creature has become troublesome and is acting out. The only thing which can be done is to remove her species confusion by reducing her to her animal state and making her “safe” around humans. She cuts Dren’s clothes off, removes her makeup and then amputates Dren’s tail without anesthesia. Clive walks in and freaks out but Elsa just tells him she’ll be able to isolate the compound since Dren can probably produce the same proteins. Elsa goes to the lab and works to synthesize the protein. Clive observes Dren via webcam in her water tank and reaches out to the screen longingly when Dren does the same to the webcam (aware that he is watching). Clive freaks out and shuts the laptop off.
That morning, Barlow runs into Elsa and comments on her behavior, telling her that she and Clive are fired since they didn’t synthesize the compound. She laughs in his face and says that she just did and it’s in the fridge, leaving him shocked. Clive goes into the barn to try and find Elsa, but instead is jumped by a naked Dren. She kisses him and Clive pushes her away telling her not to. But she’s persistent and soon Clive can’t control himself. He has sex with Dren. Elsa walks in just as he finishes and she is stunned. She runs to her car and drives away before Clive has a chance to explain.
Clive drives to their old apartment and is visibly disturbed by what he did. Elsa is sitting at the kitchen table and tells him not to speak with her since he did something that shouldn’t have ever been done. She tells him that she synthesized the compound and that their jobs are safe. He tells her that they changed the rules and that the experiments over: they need to put an end to it. They return to the barn and find Dren very ill. She soon succumbs to her illness and dies.
Clive and Elsa bury her in the woods and start burning everything: the doll, the toys and blankets. As they’re doing so, Barlow and Gavin drive up. Barlow insists on seeing Dren and Clive is furious with Gavin. Gavin points out that Barlow already knew most of it since he examined the protein and found traces of human DNA in the splice. Elsa tells them that Dren is dead and to look for themselves. But at that moment Dren swoops down, carries Barlow to a tree and kills him. Clive and Elsa are horrified when it swoops down and takes Gavin as well and they realize that Dren has changed from a female to a male.
Clive pursues Male-Dren through the woods in order to find his brother, but Elsa tells him that Gavin is already dead and that they need to go. Clive drops his lantern into a pond and uses a stick to try and get it out but is dragged in by Male-Dren. Elsa fishes him out but realizes that Male-Dren is coming after her. She runs into the woods but is tackled to the ground by it. She screams to try and understand what it wants and Male Dren says that it wants to be inside of her. It rapes her. Just as it finishes, Clive drives a stick through its chest in order to kill it but it isn’t a deathblow. It advances on Clive but Elsa hits it over the head with a rock. Male-Dren looks up at her and she hesitates long enough for Male-Dren to stab Clive in the heart with its tail blade. Elsa crushes Dren’s head with the rock and collapses.
Sometime later, Elsa is with Jean and discussing the incident. Dren’s proteins and corpse will help the company file patents for years and save it from bankruptcy. Jean offers Elsa a large amount of money for her silence and then says that given the circumstances, no one would blame her if she were to stop now. It’s revealed that Elsa is pregnant (implied to be Dren’s offspring but not explicitly stated). Elsa says, “What’s the worst that could happen?” as she looks out at the city skyline. Jean tries to give her a reassuring hug.
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