Charly Gordon (Cliff Robertson) is a retarded male adult. He enjoys playing on the playground with other kids. He goes to night school, and his teacher, Miss Alice Kinnian (Claire Bloom) asks him to come to a university to take tests. She takes him through logic tests while scientists watch from behind a 2-way mirror. Charly works at a bakery, sweeping floors, where his co-workers tease him and play practical jokes on him, led by Gimpy (Edward McNally). He doesn't realize they are jokes at his expense. The next day, Alice take Charly back to the laboratory, where he races a mouse, Algernon, through a maze.
Algernon wins, and Charly is baffled - he had his lucky rabbit's foot, too. She drives him home, and asks to see his apartment, which is very plain. His landlady, Mrs. Apple (Ruth White) thinks he's fooling around with her. Alice tells him that Algernon had a special operation that made him smarter, and asks Charly if he would like to have the same operation. Alice discusses Charly's qualifications with Dr.
Strauss (Lilia Skala) and Dr. Nemur (Leon Janney). At first, the doctors are skeptical, but Alice convinces them to try the operation on Charly. Charly chats with the lab technician Bert (Dick Van Patten). Charly rides the tour bus to get around.
His friends Hank (Barney Martin), Joey (William Dwyer) and Gimpy take him to Paddy's (Dan Morgan) bar, where they play tricks on him. Depressed, he goes to the playground, and Alice finds him and tells him the operation has been approved. He's happy again, and swings on the swing. The next day, Charly undergoes the operation. When he awakes, he's no smarter, and Algernon still beats him in the maze test. Charly is frustrated, and storms off. He works off his anger riding bumper cars, then returns to his apartment, only to find Algernon has been placed in his room. He yells at Algernon, and Mrs.
Apple tells him that having a pet is a gift. Charly begins to show improvement, and beats Algernon in a maze race. Alice continues to teach him, and Charly begins to fall in love with her. Charly studies more advanced subjects, and his co-workers try to fool him, but he's now as smart as they are. Gimpy is suspicious. Charly is beginning to notice Alice is a woman, and she is beginning to become uncomfortable with him. Alice tells Dr.
Straus that she wants to resign from the program. Charly's maturation continues, and he is fired from the bakery, since his friends no longer want to be around him. Alice explains that growth can be painful. Dr. Nemur wants to continue pushing Charly's education, but Dr.
Strauss thinks he needs to mature emotionally first. Charly follows Alice and her fiance home and tries to rape him. She screams at him, calling him a stupid moron. Charly begins to live recklessly, riding motorcycles, going to parties, and smoking. Eventually he returns home, and Alice is waiting for him. They begin a romance, and Charly asks Alice to marry him.
They have a wonderful vacation and return to the seminar to present the "Algernon-Gordon" effect. Doctors Nemur and Strauss show Charly's original test footage and Charly watches from backstage. When he comes onstage, he answers questions about what is, and what is to be. Charly then turns to the audience and asks what will become of him. No one answers, and Charly tells him that he will return to his previous intelligence - he has Algernon, and has realized that the operation is only temporary. He runs from the seminar, and is chased by images of how he used to be.
Charly winds up at a bar, where a retarded busboy is made fun of by the patrons. The doctors and Alice argue over what to do about Charly, and Charly offers to use his vast intelligence to help. He begins to experience mental blocks, a possible sign of regression. His research is unsuccessful, and Alice asks Charly to marry her. He then asks her to leave. Days later, Alice sadly watches Charly playing on the playground again. The End..