Sweet Thursday is a novel and sequel to Cannery Row written by John Steinbeck. The story is set in a part of Monterey, California known as "Cannery Row". Doc comes home from the war only to find that everything in his hometown has changed and the people he knew are either dead or gone. The transformation of Doc's town leaves him lost, depressed, and without a sense of purpose. Although Doc doesn't know what has triggered this change within him, he is determined to get his life back and find happiness.

Doc is the owner of Western Biological Laboratories, he has always been content with his life until he returns from the war. Doc tries to get his life back together by gathering new specimens and instruments for his laboratory since but finds that although things are starting to look normal in his laboratory, they just don't feel the same. Doc's friends notice the change in him, and because is he is loved so much by the community, they set out to "fix" him so that he can be happy again. Doc has become intrigued with the emotional reaction of octopi and has decided to write a paper about his findings. He has become obsessed with the idea of writing this paper, however, can't get the motivation to actually write it. Day after day, he sits in his laboratory with the intention of writing the paper with the excuse that doesn't have the right equipment or specimens to write the paper.

One day, a young lady named Suzy comes to the town. Suzy doesn't know anyone, doesn't have any money, and is only carrying a single suitcase with her. Within a few hours of being in Monterey, she gets a job at the Bear Flag, the local brothel.

Doc's friends that live at the Palace Flophouse, believe their home is owned by the owner of the local grocery store, Joseph and Mary. Joseph and Mary is a very smart Mexican man that has a background in illegal business. Mack, one of the flophouse occupants realizes that if Joseph and Mary finds out he owns the Palace Flophouse he will evict them. Mack comes up with a scheme that will make Doc happy again and keep anything from happening to his home. Mack and his friends will host a raffle where the proceeds will go towards the purchase of a new microscope for Doc and the raffle prize will be the flophouse. They will then rig the raffle so that Doc wins knowing that Doc wouldn't collect rent.

Meanwhile, Fauna being a matchmaker for the girls at the Bear Flag, has made the observation that Suzy and Doc would be good together. Fauna makes Doc take Suzy on a date and they both make a connection.

Mack and Fauna decide to make the raffle party a "Snow White and the Seven Dwarfs" theme with Suzy as Snow White. Fauna arranges an engagement ceremony for Doc and Suzy but the whole thing backfires when Suzy and Doc realize that they have feelings for each other and Suzy runs away. Ironically, when Doc wins the raffle, he tells Mack that Lee Chong had already deeded the flophouse to Mack so he was the owner.

Suzy leaves the Bear Flag and gets a job at the diner. Hazel, one of Doc's loyal friends, tries to get Suzy to work things out with Doc but she refuses, saying the only way she would ever talk to Doc is if he were injured. In the middle of the night, Hazel sneaks into Doc's laboratory and breaks Doc's arm.

When Suzy gets the news about Doc, she rushes to help him. Suzy tells Doc she will make the trip to collect specimens with him. Doc realizes Suzy is the one that he has been missing in his life and that with her he is happy. Doc and Suzy both drop their pride and confess their feelings to each other. As they are leaving, Doc's friends present him with his new microscope which is actually a telescope.