Between April and June of 1828, Joseph Smith — with Martin Harris as his principal scribe — translated 116 manuscript pages of the Book of Mormon. This first portion, which Joseph called the Book of Lehi, covered the period from Lehi’s call in Jerusalem through some portion of the early Nephite history. Then it was lost.
The events are recorded in Joseph Smith’s history and developed at greater length in Saints, volume 1. Martin Harris, troubled by his wife’s skepticism and wanting to show her the work he had been doing, repeatedly petitioned Joseph for permission to take the manuscript home. After three earnest inquiries, the Lord granted conditional permission; Harris took the pages to Palmyra, broke the conditions of his oath, and the manuscript disappeared. It has never been recovered.
The Lord’s response to the loss is recorded in Sections 3↗, in which Joseph Smith is sharply reproved for fearing man more than God, and reassured that the work will not be frustrated. The fuller explanation comes in Sections 10↗: the Lord explains that the small plates of Nephi were prepared centuries earlier to serve as a replacement for whatever portion of the larger record might be lost. Nephi himself records, in 1 Nephi 9:1–6↗, that he was commanded to make a second smaller set of plates “for a wise purpose in him.” Mormon, abridging the records nearly a thousand years later, notes the same thing — that he is including the small plates of Nephi “for a wise purpose” without yet knowing what that purpose would be (Words of Mormon 1:6–7↗).
The published Book of Mormon, then, begins not with Mormon’s abridgement of the larger plates (which it would have, had the 116 pages survived) but with the small plates’ direct account — 1 Nephi through Omni — and then proceeds to Mormon’s abridgement. The providential pattern is striking: a replacement prepared more than a thousand years before it was needed, for a contingency the Lord foresaw. The lost 116 pages are a permanent gap in what readers have; the Book of Mormon as we now have it carries the design that anticipated the gap.