In chapter thirty-seven, Aunt Sally begins to notice items have gone missing from her house. Her husband tries to blame it on the rats and vows to stop up the holes in the basement to prevent them from getting in the house. Aunt Sally isn't convinced that rats could steal an entire sheet or a candlestick. Tom and Huck feel bad, so they decide to help by filling in the rat holes. They also tried to put a few things back that they had taken.

Tom begins chapter thirty-eight by explaining to Jim how he needs to create a coat of arms. He explains that this usually consists of a picture with a saying that represents Jim. Tom suggests a few examples, and he wants Jim to scratch it into a rock. However, Huck and Tom are unable to carry the rock into the cabin, so they must unchain Jim, lead him out of the cabin, and have him assist in carrying the grindstone back to the cabin where they help him rechain himself to the bed. Then Tom mentions that prisoners always have spiders and snakes surrounding them, so they'll need to fetch some of those for Jim along with some rats. Tom wants Jim to tame them by playing music to them. He also wants to bring Jim a flower, which he can water with his tears. Jim is having trouble putting up with all this nonsense, and Tom is losing patience with Jim questioning each step of the process.

In chapter thirty-nine, Huck and Tom unstop one of the rat holes and set up a trap, which catches about fifteen rats for them, which they proceed to hide under Aunt Sally's bed. Later her son found the rat trap and opened the door, which led to Aunt Sally standing on the bed, screaming. It took Huck and Tom another two hours to catch a new batch, which they added to the bunch of spiders, bugs, and frogs they had to found to bring to Jim. Then they put some garter and house snakes in a bag but didn't tie it tight enough, so they escaped all over the house as well. Aunt Sally beat them both for sending such critters all over her house. All of the critters were sent to live with Jim in the cabin, who claimed that there was no room left for him. In the mean time, Silas Phelps had been trying to find Jim's owner in New Orleans according to the fake reward poster that the King had given him, and when he didn't get a response, he decided to advertise in the St. Louis papers, which Huck was worried Miss Watson might see. Tom said it was time to anonymous letters to the family warning them that something was up. For some reason Huck had to dress like a servant girl to deliver it. Then the family became very nervous. Tom wrote a second letter from "an Unknown Friend," which said they were planning to steal Jim that night. Huck, of course, didn't understand why they had to give away their plan, but Tom explained that they always do it that way in the books.

In chapter forty Huck is sent into the cellar for a hunk of butter that Tom requires for their plan when he runs into Aunt Sally who questions what Huck is doing up at that time of night. She then forces him to stay in the living room where Huck finds fifteen farmers with guns who have gathered to help catch the people coming to steal Jim. Huck starts sweating, he's so worried about being shot, which causes the butter to melt under his hat where he hid it. Aunt Sally sees it and thinks his brain is oozing out until Huck explains it to her. Then she sends him up to bed. Huck rushes right out his bedroom window to find Tom and tell them they need to leave immediately. They hid with Jim in the lean to then started to sneak away, but the men heard them and fired their guns, so the three of them took off running to the canoe. When they reached the canoe, they realized Tom had been shot in the calf, so Huck offered to go find a doctor. Jim said he would hide in the woods when the doctor came so that he wouldn't be caught again after he had just regained his freedom.

These chapters show the crazy imagination of Tom Sawyer along with his innate racism. He doesn't truly see Jim as an equal; otherwise, he wouldn't be treating him so cruelly. Even though he is helping him escape slavery, Tom sees it more as a game where he can make Jim do whatever he says. Huck disapproves but is willing to put up with it as long as Jim is eventually freed.