We open in a dusty field with a man in a beekeeper suit searching for a wasp nest. Dr. Eric Zinthrop (Michael Mark) spots a particularly nice specimen. He applies smoke to the nest, breaks the branch off and places it in a box. Zinthrop returns to the beehive area and talks to one of the beekeepers tending one of the many hives. A company representative, Mr. Barker (Gene Corman), arrives at the grove and chats with one of the beekeepers, Renfro (Aron Kincaid) about production quotas and Zinthrop's research, or lack of a report. He is escorted up to Zinthrop's shack. Zinthrop explains his research to Barker on extraction of royal jelly from wasps and its effects on aging. As an example he shows Barker two dogs, the same age, yet one looks like a puppy. Zinthrop has been injecting the smaller one with his formula. Barker fires Zinthrop on the spot.
Janice Starlin (Susan Cabot) is chairing an executive staff meeting at Janice Starlin Enterprises, a cosmetics company in New York. Sales for the last fiscal quarter have dropped 14.5 percent. She notes that her competitors have not experienced a corresponding drop and she demands an explanation. An uncomfortable silence accompanies a pan around the conference table. She starts with Paul Thompson (Roy Gordon). He is responsible for Public Relations. Bill Lane (Anthony Fred Eisley) expecting he will be next for a snarky reply from Janice volunteers his assessment. He blames her. He gets up and explains that sales fell because Janice used to be the face of advertising, but because of advancing age, stopped. The meeting is adjourned when her secretary, Mary Dennison (Barboura Morris) reminds Janice that she has an appointment with Zinthrop. Before her appointment, she asks to meet with Arthur Cooper (William Roerick) to discuss royal jelly. She asks his opinion about the rejuvenating effect a more powerful version from the queen wasp might have on humans. Cooper advises, strongly, against it. Zinthrop is finally told he can meet with Miss Starlin. He enters her office and is told curtly that she doesn't have much time for him. He tells her he has ten maybe fifteen years for her. She demands proof, so they go down to a lab for a demonstration. He shows her two old guinea pigs in a cage. He injects one and places it back in the cage. To her amazement, one is now younger. She tells Zinthrop, "I don't believe it, it isn't not possible." Anticipating her response, Zinthrop injects his formula in the second lab animal, with the same results.
After the demonstration, Janice is convinced of the potential and asks Zinthrop about terms. Zinthrop asks for a lab and a percentage, but he demands "full credit for my discovery, that is most important to me." Janice agrees to all terms and makes one of her own--she must be the experimental subject, in her own words, "Janice Starlin will be your next guinea pig." She also confides in Zinthrop that she's about to lose her business if his formula doesn't succeed.
At her next staff meeting, she introduces Mr. Zinthrop to her senior staff and explains that he has a free hand on his special project and will report directly to her. He begins his work in earnest. Bill Lane and Mary Dennison debate Zinthrop's motives and expertise. Lane believes Zinthrop is a con man. They leave for the afternoon and have a date for dinner together. They are joined by Cooper, who believes Zinthrop is more dangerous than a mere confidence man--he thinks Zinthrop is a quack, and that can be fatal.
Secretaries Jean Carson (Carolyn Hughes) and Maureen Reardon (Lynn Cartwright) are gossiping when Zinthrop inquires as to Miss Starlin's availability. He wants her to visit the lab, but is told she is in conference and not to be disturbed. Zinthrop returns to the lab and sometime later Janice arrives and is shown another test animal--a cat. But this time it is a kitten and Zinthrop is satisfied that the formula is ready and it is time for Janice to get her first injection.
Janice is talking on the phone with the accounting department. She directs Mr. Gleason to pay all invoices and not to question any of Zinthrop's expenditures. She doesn't know Mary is eavesdropping on the other line. Mary immediately calls Bill Lane to tell him that Zinthrop just spent $2,300.00 on enzyme extracts.
By the third week of her treatments, Janice is becoming impatient and can't see any progress in her own rejuvenation, despite Zinthrop's examination and assurance that the treatments are working. She asks about an increase in dosage, but he tells her that patience and care is required. He tells her that he has a newer, stronger formula, but that is isn't quite ready yet, and will need to be a topical treatment.
Mary takes something from Janice's desk, then goes to lunch with Lane and Cooper to disclose the contents of Zinthrop's letter to Janice Starlin. Cooper wants to do a more thorough investigation of Zinthrop.
That evening, Janice leaves her office and goes to Zinthrop's lab. She secretly injects herself with the new, but untested, formula. The next morning when Janice arrives for work she is visibly younger, even Maureen, one of the secretaries, comments. At her next senior staff meeting she unveils her new look and a new campaign for Janice Starlin Cosmetics.
Zinthrop arrives in his lab and hears a crazed cat. It attacks him, he kills it. He realizes the formula isn't ready for use, but is unaware Janice has been secretly using it. He is also unaware that Cooper plans to break into his lab and examine his notes. After disposing of the cat's body, Zinthrop leaves the building for lunch. While crossing the street he is struck by a car and is seriously injured. While he's out Cooper does break into Zinthrop's lab, examines his notes, but is interrupted from a further investigation by the arrival of Janice Starlin knocking at the locked door. She uses her key to enter the lab, but discovering his absence, leaves. Cooper continues his snooping.
When Zinthrop fails to return from lunch, Janice meets with a private investigator, Les Hellman (Frank Gerstle) and demands that Zinthrop be found and that time is crucial. She won't explain why. She has very little personal information to aid the investigator, then remembers his letter in her desk. She checks her desk, but can't find it. She remembers something Mary said and calls and has her return to the office and explain taking the letter. The investigator canvases the city and finally locates Zinthrop in a local hospital. The doctor (Roger Corman) explains it was a head injury and there was brain damage, but they can't determine the extent at the moment. Janice demands the best doctors available and that she will be financially responsible. Three days later Janice tells Cooper that if Zinthrop fails to recover he is to take over the lab. Janice sneaks down to the lab for another treatment. Cooper examines Zinthrop's notes and realizes the formula has a problem. When he examines the vials he is interrupted by a buzzing sound and the appearance of a wasp headed female figure. He tries to attack it, but it overpowers him. The night watchman (Bruno Ve Sota) begins his evenings rounds. Janice, now herself again, finishes injecting herself with the experimental formula and notices the vial is almost empty.
At the next executive staff conference, Janice is taken ill. She ends the meeting early. She realizes she is now dependent on the injections, like a drug addict, and the supply is running very low. She decides to move Zinthrop into the building while he recuperates, and has a rollaway cot placed in her office so she can be close to him. A private nurse accompanies Zinthrop to an office that has been converted into a hospital room. Cooper's absence has been noted by Bill Lane and Mary Dennison, but Janice explains that Cooper is entitled to take a little time for himself.
The night watchman hears something outside Zinthrop's lab during his rounds. He enters and is attacked. His scream wakes Zinthrop, but the nurse convinces him it was just a dream, although she also heard the scream. The next morning Janice is told about the disappearance of the night watchman, but she dismisses the discussion. Bill and Mary break into the lab while Janice meets with Zinthrop. His brain injury and memory loss frustrates him, as he tries to explain that there is something important he must tell Janice. Bill and Mary find Cooper's pipe and are now convinced he's in the building and possibly dead, along with the night watchman. As Zinthrop looks at her, Janice metamorphoses into the wasp creature. The nurse walks in at that moment, and the wasp creature attacks and kills her. Zinthrop collapses back in bed. Bill and Mary head for Zinthrop's room to question him and notice the room in some disarray and the nurse missing. Janice heads back to the lab for another injection. Mary goes up to warn Janice and Bill remains with Zinthrop. Mary implores Janice to call the police. Meanwhile Zinthrop explains to Bill that Mary is in great danger. Janice is the wasp creature and will kill Mary. Mary tells Janice what she and Bill have discovered so far. As she does, Janice transforms into the wasp creature and attacks Mary. They struggle and Mary is dragged off to the lab to be devoured. Bill takes the stairs up to the lab, as Zinthrop waits for the elevator for the same destination. Bill confronts the wasp creature in the lab, and as Zinthrop enters the lab is set upon by the wasp creature. Bill picks up a lab stool and attacks the creature. Zinthrop grabs a bottle of carbolic acid and throws it. The bottle breaks hitting the creature on the head. As the acid fumes and the wasp creature is temporarily dazed, Bill grabs the stool again and pushes the creature out the window. Bill finds Mary in the lab, dazed but otherwise well. We close gazing on the wasp creature's face while it transforms into a shot of a bee colony.