Charlie Gordon, a 32-year-old man with an intellectual disability, works as a janitor at a bakery and attends classes at the Beekman College Center for intellectually disabled adults. He is selected for an experimental procedure to increase his intelligence. Charlie writes progress reports to track his thoughts and experiences. Preliminary tests and races with a mouse named Algernon are conducted. Charlie's sister gives permission for the experiment, and the surgery is performed. Charlie is disappointed when he doesn't immediately become smarter. He continues his studies and longs to return to work. Charlie faces challenges at the bakery and neglects to return to the lab, but he is given a device to improve his intelligence while sleeping. Therapy sessions and social interactions reveal both frustrations and improvements in his intelligence.
Charlie's surgery goes well, but he is disappointed by the gradual progress in his intelligence. He resumes work at the bakery and continues tests at the lab. Charlie faces mockery from coworkers but receives support from Mr. Donner. He recalls memories from his past and participates in therapy sessions. Charlie attends a party where he is mistreated, but he achieves his first victory over Algernon in the maze. Charlie's reading and spelling skills show improvement.
Charlie impresses his coworkers with his ability to operate a dough-mixing machine. He receives a promotion and raises, but encounters resentment from some coworkers. He continues to learn and recall
memories from his past, although a party incident leaves him confused and embarrassed. Charlie becomes aware of people mocking him and realizes his emotional development lags behind his intellectual growth.
Charlie further enhances his performance at work, which leads to increased resentment from his coworkers. He recalls a kind action from Gimpy and recognizes the changing dynamics with his colleagues. Charlie is allowed to keep some progress reports private temporarily. He engages with intellectual topics at the college and forms friendships. Charlie spends time at the library, reading extensively. Childhood memories of his parents' arguments and his emotional reactions to them resurface.
Charlie develops romantic feelings for Alice and goes on a dinner and movie date with her. He feels confused and scared to act on his emotions. Frustration persists as his increased intelligence doesn't bring back much of his past memories. Alice comforts him and suggests that he needs more time to develop. Charlie wonders if his childhood memoriesof inappropriate situations are hindering his ability to pursue a relationship. While attending a concert with Alice, Charlie believes he sees a teenage boy exposing himself, but he questions whether it was a hallucination.
At the bakery, Charlie discovers that Gimpy has been stealing money. He struggles with the moral dilemma of confronting Gimpy or staying silent. Dr. Strauss suggests Charlie doesn't have to get involved, but Alice leaves the decision up to him. Charlie confronts Gimpy and offers a deal to stop the theft in exchange for not reporting him to Mr. Donner. Gimpy resents Charlie's interference, and Charlie is later fired from the bakery due to complaints from other workers. Feeling superior in his knowledge, Charlie visits Alice and criticizes her life choices, causing tension between them.
Charlie loses interest in the lab work and neglects to submit some progress reports. He becomes annoyed with Dr. Nemur and Dr. Strauss. Nightmares and memories of his younger selves watching him disturb him. Childhood memories resurface, including his sister's frustration with his disability. Memories involve his parents arguing about his treatment, with his mother wanting him to become "smart" and his father advocating acceptance. Charlie realizes his motivation for intelligence improvement is linked to his mother's fear, guilt, and shame.
Charlie visits the Beekman Center and encounters confusion from former classmates and anger from Alice due to his attitude. He defends his behavior by claiming that people have always treated him condescendingly in the past. Charlie wanders the city and meets a pregnant woman who propositions him. Shocked, he grabs her shoulder, causing her to scream for help. Charlie hides in the bushes while people search for him.
Charlie and the research team attend a conference in Chicago to share their findings about the operation and Charlie's development. Charlie recalls being taken to a specialist, Dr. Guarino, as a child, who attempted to treat his intellectual disability. During the conference, Charlie becomes angry as he feels objectified by Nemur and others, who discuss him as a specimen rather than a person. At a party, Charlie challenges Nemur's ideas and shares new research that even Nemur is unaware of. The conference presentations bore Charlie, and he gets the urge to free Algernon, the mouse, from his cage. Charlie manages to escape with Algernon, unnoticed, and returns to New York with him.
The media widely covers Charlie's escape with Algernon, whohas started displaying erratic behavior. Upon questioning Charlie's family, he obtains the address of his mother and sister, Norma. Looking at a photo of them, Charlie remembers his mother's affection toward Norma while rejecting him. He also recalls his parents arguing about this issue. Instead of going home, Charlie stays at a hotel and rents an apartment. He meets his neighbor, Fay Lillman, an artist who embraces a carefree
lifestyle involving drinking and dancing. Charlie is immediately attracted to Fay on a sexual level.
Charlie tracks down his father, Matt, who owns a barbershop in the Bronx. He wants to introduce himself to his father, but Matt fails to recognize him. While Matt shaves him, Charlie reminisces about the night his mother forced Matt to take him away from their home to live with Uncle Herman. The memory overwhelms Charlie, and he leaves the barbershop without revealing his true identity.
One night, Fay visits Charlie's apartment after an argument with another man. They nearly engage in sexual activity, but Charlie stops when he envisions his younger self watching them. Fay stays the night, and they both get drunk. The following morning, Fay informs Charlie that while he was intoxicated, he regressed to a childlike state, which she found amusing. Charlie wanders through the city and witnesses a young man with an intellectual disability being ridiculed for dropping dishes in a diner. In response, Charlie shouts at the onlookers, emphasizing that the young man is a human being. This triggers memories of others mocking him in the past.
Charlie decides to return to the laboratory and dedicate himself to advancing the intelligence of individuals with intellectual disabilities. He also visits Alice, who had been concerned about his disappearance. Charlie confesses his romantic feelings to Alice, leaving her both flattered and unsettled. In an attempt to overcome the hallucination of his younger self, Charlie suggests having sex with Alice while imagining she is Fay. However, the plan fails. Alice consoles Charlie, acknowledging his progress, but he leaves feeling upset. When he returns home, he finds Fay in her apartment. They drink and have sex, despite Charlie still seeing his younger self observing them. Their relationship continues, and Charlie no longer experiences the hallucination. However, he begins to feel exhausted from their frequent nights of dancing and partying.
Charlie returns to the laboratory, much to Nemur's annoyance, as Charlie had approached the Wellberg Foundation andstipulated that he would only come back if allowed to work on his independent research. The team observes that Algernon has started to forget what he had previously learned and is displaying erratic behavior. During a tour of the
lab, Charlie is informed that if Algernon dies, he will be incinerated. Charlie adamantly refuses to accept this plan and insists on being given the opportunity to bury Algernon in the event of his death. Charlie also learns that his fate had been discussed by the team and his sister, Norma. If the operation fails and he cannot care for himself, Charlie will be sent back to the Warren State Home and Training School instead of working at the bakery. Witnessing Algernon's decline, Charlie becomes aware that the operation may also be failing him.
Charlie visits the Warren State Home, contemplating the possibility of returning there in the future. The head of the home shows him around, highlighting the residents' freedom of movement. However, Charlie is moved when he learns about the challenges faced by individuals with severe disabilities from the woman in charge of the buildings. He also observes two residents seeking comfort in each other's arms, as they long for human contact.
Back at the lab, Algernon refuses to participate in the tests. Charlie continues his partying with Fay but begins to feel exhausted. Unexpectedly, Alice and Fay meet at Charlie's apartment and get along well. However, an argument ensues between Charlie and Alice regarding his excessive drinking. Although Alice relaxes upon learning that Charlie hasn't revealed his true self to Fay, he starts spending more time at the lab to avoid going out with Fay as frequently. Charlie becomes obsessed with his work, feeling the need to accomplish a lifetime's worth of innovation before his intellect deteriorates.
Charlie studies Algernon's erratic behavior and believes he is close to discovering the cause. Overwhelmed by his workload, he attends a party at Nemur's house where he discusses his research with the other guests, including Mrs. Nemur. The effect of alcohol takes its toll on Charlie, leading to a stern conversation with Strauss about his behavior. An argument ensues between Nemur and Charlie, with Nemur accusing Charlie of being unfair and ungrateful, while Charlie argues that the others have failed to treat him as a human being. Charlie begins to feel like his younger self, engaging in a conversation with his reflectionin a mirror. He realizes that Nemur is partially correct about his behavior.
Charlie writes a letter to Nemur, explaining his theory of "The
Algernon-Gordon Effect: A Study of Structure and Function of Increased Intelligence." He theorizes that artificially induced intelligence declines at a rate proportionate to the speed at which it was gained. Nemur confirms the validity of Charlie's hypothesis. Charlie is fully aware that this theory applies to himself, and he starts preparing for his intellectual decline, noticing signs of absentmindedness. When Algernon dies, Charlie buries him and places flowers on his grave.
A few days later, Charlie visits his mother, Rose, and Nemur. He is taken aback by the deteriorated state of their old neighborhood. Initially, Rose fails to recognize him, but after a moment, she lets him in. They discuss the operation and Charlie's development. Rose fusses over household tidiness and scolds Charlie about the electric bill, exhibiting signs of dementia. During moments of clarity, Rose expresses gratitude for her prayers being answered regarding Charlie. Charlie attempts to leave before Norma arrives, but they inevitably meet. Norma is overjoyed to see Charlie and apologizes for how she treated him in the past, explaining the stress she was under at the time. They reconcile and embrace, while Charlie understands Rose's behavior.
Charlie's mental decline has begun, causing him to feel depressed. He plays loud music at home and becomes easily irritable. He confides in Strauss about the failed meeting with his father, becoming annoyed with Strauss and experiencing hallucinations. Charlie feels a sense of unity with the universe but perceives a cave-like wall with a narrow opening blocking his path. He informs Strauss of his desire to end their therapy sessions.
The regular tests at the lab become increasingly challenging for Charlie. He struggles to recall basic information and contemplates quitting the tests, growing annoyed by the team members' reassurances. Reading becomes difficult, and one morning, Alice appears at his apartment. She states that she has come to spend their remaining time together. Initially brushing her off, claiming he needs time for work, Charlie realizes that embracing Alice doesn't trigger panic or hallucinations. Instead, he feels a genuine connection and shares a loving sexual encounter with her. Alice decides to live with Charlie in his apartment.
Both Charlie and Alice are aware of the limited time they have left. Charlie's intellectual struggles persist, leading to a decline in foreign languageskills and difficulties comprehending his own research. His coordination deteriorates, irritability increases, and he finds solace only in television. Eventually, Charlie and Alice have an argument, causing her to move out. Strauss and Charlie's landlady check up on him from time to time. Fay installs a lock on her door to prevent Charlie from visiting. Charlie continues to visit Algernon's grave to place flowers.
Alice and Strauss come to visit Charlie, but he refuses to let them in. Despite his declining condition, Charlie continues to write progress reports, simplifying them due to his diminishing cognitive abilities. He returns to Mr. Donner's bakery and asks for his job back, facing bullying from one of the new workers. However, Joe Carp, Frank, and Gimpy assure Charlie that they will support him.
Charlie resumes attending his class at the Beekman College Center, surprising and upsetting Alice. Feeling remorse for causing trouble at the bakery and upsetting Alice, he decides to admit himself to the Warren Home. He concludes his progress reports by requesting someone to place flowers on Algernon's grave.