Law student Allan Mann awakes early in the morning to exercise. Leaving his girlfriend, Linda Aikman in bed, Allan goes running with a backpack of bricks to weigh him down. A German Shepherd startles Allan to back into the street, and he is hit by a truck. At the hospital, Dr. John Wiseman operates on Allan's spinal cord. However, the accident has paralyzed Allan from the neck down.Later, Allan's mother, Dorothy, holds a surprise party to welcome him home after a lengthy hospital stay. Before Allan arrives, Linda discreetly packs her belongings before she breaks up with him. When Allan arrives in a wheelchair, Linda apologizes for not visiting him in the hospital. In the kitchen, Linda leaves a telephone message for Geoffrey Fisher, Allan's friend and former roommate, asking why he isn't at the party.Meanwhile, Geoffrey is at the Experimental Medicine laboratories at the local university. Removing a human brain from a canister labeled "Live Human Tissue", he creates a serum with the brain matter after flash freezing it. From a wall unit of caged research monkeys, Geoffrey frees a Capuchin monkey from cage number 6 and injects the serum into her. Later, Geoffrey drives to Allan's house. Finding his friend in a hospital bed, Geoffrey apologizes for missing the party. Allan despondently tells him that he thinks Linda broke up with him because she can't handle his quadriplegic state.Returning to the university, Geoffrey chases off students spray-painting the Experimental Medicine entrance with accusations of torturing animals. Afterward, he meets with Dean Harold Burbage, head of the Experimental Medicine department, who inquires about Geoffrey's human memory cells serum and the test monkey's reactions. However, Geoffrey advises Burbage it's too early to tell. Returning to his lab, Geoffrey injects more serum into the monkey, but not seeing a change in her intelligence, he thinks she needs to interact with more humans.The following morning, Geoffrey visits Allan, but finds his friend attempting suicide by suffocating under a plastic dry-cleaning bag. After Geoffrey calls an ambulance, Allan returns under Wiseman's care. Suddenly, Geoffrey sees Linda and asks why she is there. Wiseman stands next to her and tells Geoffrey they are dating. Upset, Geoffrey returns to Allan's room, not telling him about Linda.Later, Geoffrey drives to the country to meet Melanie Parker, a trainer of helper primates who assist quadriplegic individuals. Learning there are no monkeys currently available, Geoffrey offers to donate one from his lab. Melanie agrees after Geoffrey lies and says the primate has not been exposed to abnormal experiments. After Melanie trains the monkey, she and Geoffrey go to Allan's house and introduce him to his new companion, "Ella". As Melanie and Allan's live-in nurse, Maryanne Hodges, bring in Ella's cage, Geoffrey tells Allan that he smuggled her out of his lab, but not about the serum injections. Melanie visits several times to equip Allan's house and wheelchair to help him communicate with Ella, with voice commands and laser pointers. Through bonding with Ella, Allan feels better, and is able to return to law school. One afternoon Allan shows Geoffrey how Ella can make telephone calls with assigned punch cards. Geoffrey marvels at Ella's increasing intelligence as she calls his lab, but becomes upset when he hears Dean Burbage answering the telephone. Furious at Burbage breaking into the lab, Geoffrey goes to confront him. Instead, Burbage asks about the missing monkey. Geoffrey says the monkey died ten weeks ago, but it was not the animal involved with his brain matter experiment.Later, after nurse Maryanne Hodges has put Allan to bed, they get into an argument over Ella. When Maryanne leaves, her bird, "Bogie", flies in and starts pecking at Allan's face. Hearing cries for help, Maryanne returns, but almost pushes Allan onto the floor. Angry, Allan orders her to get rid of Bogie, but Maryanne refuses. Reacting to Allan's anger, Ella escapes her cage and sneaks into Maryanne's room. Ella kills Bogie and places the bird in Maryanne's slipper.The following evening, after Allan's mother appears for a surprise visit, Maryanne accuses Allan of sending Ella to kill Bogie. Though denying it, Allan spitefully says the bird deserved to die. Dorothy tells her son that Maryanne is quitting, and she will be moving in to help. While assisting Allan into bed, he sees his hand move on its own, but Dorothy blames his imagination. That night, Allan dreams he is in Ella's body, running outside. The next day, Allan tells Melanie and Geoffrey about the dream. To reassure Allan, they inspect the house to see if Ella could go outside. In the attic, Geoffrey sees a torn window screen, but doesn't tell Allan. He also puts down Allan's idea of having a telepathic connection with Ella.Later, in light of his hand moving, Melanie drives Allan to the office of Doctor Williams to get a second opinion about his paralysis. Allan learns that his condition may be congenital and that Wiseman might have noticed it during surgery, but didn't, as he was focused on the effects of the accident. Afterward, Allan vents to trainer Melanie about Wiseman's incompetence. Disturbed by his anger, Melanie reminds Allan that he may be able to walk again, but Allan continues to fixate on Wiseman.At home, Allan calls Wiseman, but learns the doctor is gone for the weekend. Insisting it's an emergency, he is given another telephone number. However, Allan realizes the number is to a cabin owned by Linda. Reacting to Allan's rage towards Wiseman and Linda, Ella sneaks away to the cabin, and sets it on fire, killing Linda and Wiseman. After learning of the fire, Allan tells Geoffrey and Melanie that Ella must have started it because he wished for something bad to happen to Linda and Wiseman.At Allan's insistence, Geoffrey takes Ella back to his lab. Melanie invites Allan to stay at her house for the weekend. As Melanie helps Allan out of his wheelchair onto the bed, he tries to kiss her, but misses. He looks away ashamed, but Melanie continues to help him onto the bed. Afterward, she lies on top of Allan, kissing and making love to him. Two days later, Melanie and Allan meet again with Doctor Williams, who agrees to operate on Allan if he is able to prove that he can move his body on his own. Returning to Allan's house, Dorothy expresses her displeasure on not knowing where her son was. Allan apologizes for his recent attitude, saying Ella was the cause of his anger.In his lab, Geoffrey starts to believe that perhaps the serum is causing Ella and Allan to have a mental connection, and decides to take the remaining serum himself. As Geoffrey passes out, the lab monkeys escape and tear apart the lab. Coming out of his haze, Geoffrey sees Ella is gone. Filling two syringes with poison, Geoffrey drives in the rain to warn Allan. Ella enters the house from the attic, and Allan feels the monkey's angry presence. He yells hateful accusations at his mother. Upset, Dorothy slaps Allan before leaving him in bed. Knowing the anger is connected to Ella, he calls out warnings about the monkey, but Dorothy intentionally ignores him and takes a bath. As Dorothy sits in the bathtub, Ella throws a plugged-in hairdryer into the water, electrocuting her. Geoffrey arrives and, after helping Allan into his wheelchair, confesses that he is responsible for genetically manipulating Ella. Naturally, Allan is furious at Geoffrey for lying to him and for being a Guinea Pig to him the whole time. Even though Geoffrey insists he was trying to help Allan in the beginning, Allan doesn't believe him.Suddenly, Ella appears and cuts Geoffrey's hand with a straight razor. Geoffrey pulls out the poison-filled syringe and goes after her as she chews the wires of the ringing telephone. However, Allan Manages to Get the kitchen Phone off the hook. Melanie is on the other and upon hearing Ella trying to smack the receiver down, realizes that Allan's in trouble. Ella grabs one of Geoffrey's syringes and injects him with poison, killing him. Allan attempts to escape, but is unable to do so as Ella turns off the power and holds him hostage. Melanie arrives, and upon finding the doors locked and Allan inside without power, throws a rock into his window, comes inside and turns the power back on. Ella attacks Melanie and knocks her unconscious. Ella tries to light Melanie on fire, but her rain soaked clothes won't ignite. Instead, Ella finds Geoffrey's other syringe and attempts to inject Melanie. Allan is able to move his right arm and turns on his tape deck to distract Ella. Hearing the music, Ella goes towards Allan. While she hugs him, Allan bites Ella on her neck and violently tosses her back and forth, snapping her neck until throwing her corpse into his tape deck.Later, Allan dreams he is being operated on by Doctor Williams. When an incision is made on his back, Ella bursts out. Awakening, Allan finds himself in the hospital after the operation. Having regained his ability to move, Allan and Melanie leave the hospital together.