Law student and athlete Allan Mann is struck by a truck and rendered
quadriplegic following surgery. As his domineering mother and uncaring nurse try helping him, Allan, now in an
SNP-equipped wheelchair, struggles to adjust. His friend, Geoffrey Fisher, suggests he get a
capuchin monkey as a
service animal to lift his spirits and help around the house. Geoffrey has an ulterior motive: He is a research scientist under pressure to produce results. Geoffrey has dosed a monkey in his lab with a serum to boost its intelligence, and believes the serum's effects will be amplified if the monkey is around humans. Geoffrey enlists Melanie Parker, a specialist in training
helper monkeys. Geoffrey provides the Capuchin he has been experimenting on, claiming it is normal. Allan names the monkey Ella, and he and Melanie work closely training her. Initially, Ella is a huge help to Allan. Allan grows very close to Melanie. Allan grows short-tempered and resentful. Ella, too, becomes aggressive. Allan dreams of running through the grass at night, and believes he has a
telepathic link with Ella, whom he suspects can escape the house. Geoffrey finds evidence confirming this but, pleased with Ella's intelligence, does not tell Allan and Melanie. After the pet bird of Allan's uncaring nurse pesters him, Allan wishes it was gone - that night, Ella kills the bird and leaves it in the nurse's slipper, causing the nurse to quit in a rage. Allan gets a second opinion about his paralysis and discovers he may have been misdiagnosed; another surgery may enable him to walk again. Before attempting the risky surgery, the doctor needs Allan to demonstrate some ability to willingly move an appendage. Rather than feeling happy at this news, Allan is furious with his original surgeon who is incompetent and egotistical as well as now dating Allan's former girlfriend. He fantasizes about burning the man's cabin down. That night, Ella kills the surgeon and his girlfriend by burning down the cabin with a pack of stolen matches. After seeing the fire in a vision and having it confirmed by his mother, the horrified Allan believes that Ella is carrying out his dark impulses. He also realizes that when he is around Ella, he becomes easily enraged. Allan demands that Geoffrey take Ella away. With Ella gone, Allan becomes more relaxed; he spends the weekend at Melanie's house and the two have sex. Allan returns home and tries to mend bridges with his mother, but feels a burst of rage at her utter contempt of Melanie. Allan realizes that Ella must be nearby. Allan's mother ignores his desperate warnings that she leave, and instead takes a bath. Ella kills her by dropping a hairdryer into the tub,
electrocuting her. Allan briefly answers a call from Melanie before Ella disables the phone. Realizing Allan may be in trouble, Melanie departs for his house. Geoffrey arrives and confesses that Ella was an experimental subject. The enraged Allan demands Geoffrey leave, upset at being used for an experiment yet concerned for his safety with Ella around. Instead, Geoffrey pursues Ella around the house, intending to
put her down with one of the
syringes of
sodium pentobarbitone he has brought. Ella gets hold of a syringe and injects it into Geoffrey, killing him. Ella returns to Allan, who is filled with self loathing because he believes that Ella is acting out his own impulses. He screams at Ella, but the monkey responds by
urinating on him. Melanie arrives and Ella attacks her. This convinces Allan that Ella is not simply carrying out his desires, but is acting as a jealous romantic partner. Melanie falls and is knocked unconscious. As Allan rages at Ella, she ignores him and prepares to inject Melanie with Geoffrey's other syringe. Allan calms himself and moves his right arm to engage his tape deck. As peaceful music plays, he lovingly summons Ella to cuddle close to him, and she complies. When Ella comes near enough, Allan bites her on the neck and slams her body against the back of his wheelchair and into his tape deck, killing her. Now that he has demonstrated a voluntary movement, the surgery can be performed and is a success. Melanie arrives at the hospital to pick him up, and Allan stands up from his wheelchair and gets into the van with her. ==Cast==