Tristram picks up where the last chapter left off, undertaking now "to explain the nature of the perplexities in which my uncle Toby was involved" in his attempts to tell the story of his war wound. Toby's trouble was that the military maneuvers in question were so intricate and technical that nobody could understand him; indeed he sometimes even confused himself as well. It occurs to him now to get a large map of the environs of Namur, which relieves him of his difficulty and also sets him off on his hobby-horse.
Tristram informs the reader that his book is to be a "history-book" in the same way that Locke's Essay Concerning Human Understanding is a history-book--that is, as a history "of what passes in a man's own mind." He goes on to attribute the obscurity in Toby's battle descriptions not to any kind of mental confusion, but to a confusion in language itself: the "unsteady uses of words" that predominates especially in the technical jargons of specialized fields.
Captain Toby's obsession with his map grows, and he launches into a detailed study of fortification and military science that becomes his ruling passion. He soon grows restless for his recovery. Tristram, after reminding us that he still means to resume the interrupted conversation from Volume 1, Chapter 21 (when he cut Toby off at "I think --"), proceeds with the story of his uncle's sudden desire to leave the sickbed: Corporal Trim, Toby's servant, had planted the suggestion in his master's mind that they should move to the country in order to construct a replica, built to scale, of the battle site and fortifications. This idea pleases Toby so intensely that he can hardly wait to begin.
Tristram resumes the scene by the fireside on the day of his own birth, and Toby finishes his long-delayed sentence by suggesting that they ring the bell to inquire about all the noise upstairs. The labor has begun in earnest; Susannah runs for the midwife, and Walter sends Obadiah to fetch Dr. Slop. Speculating about Mrs. Shandy's preference for the midwife, Toby suggests that it might be a question of modesty. Walter challenges him on this point, and Toby defers, admitting that he knows nothing about women. He alludes to the unfortunate outcome of his affair with Widow Wadman as evidence of the fact. Walter begins to hold forth about the right and the wrong end of a woman, but is interrupted by a knock at the door.
Obadiah and Dr. Slop have arrived. Tristram reflects on the complications of calculating time in a narrative where events are happening simultaneously, or in comparing narrative time with lived time. He first claims that it has been an hour and a half since Obadiah left on his errand--plenty of time to return with the doctor. He then argues, from the other side, that no more than two minutes, thirteen and three-fifths seconds could possibly have passed. Finally, he offers the conjecture that years have passed, since all the stories of Uncle Toby's military career and invalidism have intervened since the birthday was first mentioned. His imaginary critic remains unpersuaded, so Tristram closes the matter by revealing that Obadiah actually ran into Dr. Slop just outside the house, in a collision that sent them both into the mud.
Obadiah is sent back out to fetch the doctor's tools, which the doctor has left at home. Toby has been put in mind of Stevinus, an engineer and writer on fortifications; he explains the connection, which seems illogical to everyone else. Walter insults him for his doggedness and stupidity. Tristram relates that Toby's feelings were hurt, but that he "was a man patient of injuries." He goes on to tell a sentimental anecdote about how Toby "scarce had heart to retaliate upon a fly," and attributes whatever goodwill he himself has learned to the early impression of his uncle's gentleness and humanity. Walter, seeing Toby's serene countenance, quickly apologizes, and the brothers are reconciled. Corporal Trim delivers a sermon on conscience (actually one of Sterne's own) that has fallen out of the volume of Stevinus. Tristram gives a minutely detailed visual description of the stance Trim assumes for this oration. The sermon proves to have been left in the book by Parson Yorick, who subsequently retrieves it.