When the doctor departs alone with the canoe, Huck must remain behind. This action leads to an entirely different and unplanned development.
Tom, who has read romantic tales, devises an elaborate scheme for the rescue of Jim, but he carries his scheme too far as he places Huck, Jim, and himself in danger. There are fifteen farmers with guns who are coming for the "cutthroats" who are trying to steal Jim. Hurriedly, Tom, Huck, and Jim slip out the escape hole and run. Unfortunately, Tom catches his pants on a splinter of the top rail of the fence he climbs, and when he pulls them loose, the splinter snaps, making an audible noise. Tom is shot in the leg. Jim refuses to leave Tom, so he stays with the boy while Huck goes for help, telling the others to hide.
Huck reaches the house of an old, kindly doctor who agrees to help Tom. The physician insists upon traveling alone in his canoe, contending that it would not be safe to transport anyone else. In truth, the doctor is rather suspicious.
Because of the doctor's departure on his own, Huck has to wait behind. As a result, things do not go as the boys planned because Jim is recaptured and Tom is brought back to the Phelps's home. There he tells his Aunt Sally that he is Sid because Huck is impersonating himself. He also reveals that Miss Watson has died, and in her will she set Jim free. Tom's aunt reacts,
So it was you, you little rapscallions, that's been making all this trouble, and turn everybody's wits clean inside out and scared us all most to death.... To think, here I've been, night after night, a--you just get well once, you young scamp, and I lay I'll tan the Old Harry out o' both o' ye!"
She then scolds the boys for meddling with Jim, but they insist he is all right. What is ironic about the delay created by the doctor is the revelation of the fact that Tom Sawyer has gone to all these elaborate plans to free Jim, when he has already been free. Still, this situation makes Huck feel better because he has wondered "how he could help a body set a negro free." This is a curious thought since Huck has considered doing the exact same thing. Thus, the doctor's going alone in the canoe creates two unusual situations.
Soon after all the confusion, Tom's Aunt Polly arrives and correctly identifies everyone. Jim is given his freedom and Huck is relieved that he is not guilty of freeing Jim illegally, despite being scolded for the trouble he has caused.
When the doctor refuses to let Huck into the canoe with him, Huck is effectively separated from Jim and Tom. The fate of his two friends is now out of his hands.
The effect on the narrative and on the reader is two-fold. Huck now has to hold his tongue and wait to find out if Tom is alright. (Tom has been shot in the leg.) He also has to wait to find out what happens to Jim (recently freed in an elaborate escape).
As Huck is made to wait, so is the reader. This creates anticipation that builds to the novel's climax wherein Tom is returned to the Phelps house and blurts out the whole tale of Tom's escape after coming back to consciousness.
This climactic scene is made possibly by the doctor's refusal to share a canoe with Huck.