Professor Jenkins, an eccentric professor, is a biology professor at the University of Wales. He is a super fat man. This reason makes him always make some small comedies at school. Once he accidentally opened the mouse cage in the laboratory, causing thousands of mice to run around the school. This angered President Ricky, who warned Jenkins and sent him to see Harry, the investor of the school. Perkins returned to the laboratory and found that his new drug experiment was successful. This drug changed the DNA of mice and made them lose weight.

Jenkins met the beautiful Miss Carlo, a female graduate student and a chemistry teacher. He liked her very much and invited her to dinner. But in the club, Perkins was teased by Ricky, the host, which made him hold his head in front of Carlo. When he got home, Jenkins drank the experimental potion, and under the action of the potion, he became a thin man. But this potion has a certain time limit, and then it will return to its original state. Biddy asked Carol to go to the club again. This time he got revenge and threw Ricky into the piano. Jenkins' assistant found out his secret, pointed out that Biddy was emotionally unstable, and advised the professor not to do it again, but Jenkins still drank the potion to date Carol. In the dining room, Biddy described the experimental project instead of the professor and got Harry's investment, but his debauchery made Carol very sad.

President Ritchie thought Biddy had solved the school's difficulties and decided to expel Jenkins. Keith drank the potion and became Biddy to see Harry. Biddy is very selfish. He wants to drink all the potions and never get fat again. But it actually killed Professor Jenkins. At this moment, the professor's assistant came and stopped him. Never put off till tomorrow what you can turned back to fat Perkins. Jenkins told the truth. Carol forgave Jenkins and encouraged him to live seriously and be kind to himself. They danced to the music.