One thing's for sure: Tristram's itching to write his crazy story. He doesn't really give a clear-cut reason, but check out the dedication for a clue: he says I live in a constant endeavour to fence against the infirmities of ill health, and other evils of life, by mirth (dedication.1). Sure, the dedication was technically written by Sterne, not Tristram. But narrator-Tristram definitely has a bit of Sterne in him—not only does he share the author's opinions, but his circumstances as well. Scholars have figured out that, a lot of times, when the narrator-Tristram talks about being sick or in pain, Sterne was also ill while he was writing. Tristram's having something of a midlife crisis, so to speak.
If you're looking for a roadmap to Tristram Shandy, take a look at the end of Book 6 (6.40). He's completely incapable of getting from point A to point B without looping wildly along the way to talk about everything but his life and opinions. It's actually a feature of quest narratives to digress—in The Odyssey, for example, Odysseus checks out from the main story he hangs out with the nymph Calypso for a while, before getting on his way. In The Aeneid, Aeneus spends some quality time with Dido before she sets him free by conveniently killing herself. But Tristram takes digression to an extreme. Instead of a narrative journey, he takes us offroading for five full books. That's dedication to digression.
In this stage of the Quest plot, the hero inches closer to his goal. Around Book 4, Tristram is finally born, and—even more important—his older brother bites the dust. What's more, Book 5 contains a biggie: the accidental circumcision. After a circumcision and a broken nose, the world's not really rolling out the welcome mat for baby Tristram. You could say that, after Book 5, Tristram realizes that he can never fully become the center of his own life because these two major signs of power—the nose and the penis—are permanently wounded. Double ouch.
Tristram gets a little sidetracked from his own story when Toby's life takes an interesting turn. If he can get through this story, he'll at least have achieved one narrative goal. Of course, he has trouble following through. Tristram runs into the same trouble he had telling his own story, and it all seems to stem* from the same cause: a missing penis. True, Tristram's penis is still there, and it turns out that Toby's is, too. But they're both suspected of missing a penis, and that seems to be just as bad as not having one at all.
*When you're writing about Tristram Shandy, everything becomes a double entendre.
Tristram can't seem to score. He fails to make the last free throw, so to speak, and the book just ends abruptly. Yeah, it's a fitting conclusion for the last of a series of failed climaxes. The story begins with an interrupted sex act and ends with a failed sex act—and a failed narrative act (see "What's Up With the Ending?" for more about this). Tristram doesn't reach his goal… unless, that is, it turns out that his goal all along was to avoid giving us the satisfaction of a plot. In that case, get ready for the Gatorade dump—he's been spectacularly successful.