The book describes the period of time immediately following the visit of Jesus Christ to the Book of Mormon peoples, in which time the Nephites and the Lamanites are all converted to the Church of Christ.
After the year AD 100, all the original disciples of Jesus had died, except for three which he allowed to live on and on until the end of human history.
The Nephi who recorded the coming of Jesus to America died, and the keeping of the plates passed to his son Amos.
One of these churches began to persecute members of the True Church of Christ, casting them into prison (but the prison then broke in two), or casting them into fiery furnaces (but the victims walked out unscathed), or casting them into a lion's den (but the victims played with the animals as if they were lambs).
The book concludes with Ammaron hiding the sacred records, which he ultimately delivered to Mormon.