Because Mosiah opens with its own opening chapters missing from the record as transmitted, it functions differently from every book before it: rather than introducing its protagonists, it assumes a reader is already partway into their story. Latter-day Saint tradition holds that the lost material was part of the original manuscript pages that went missing after being lent out during the translation process, and that Joseph Smith was directed not to retranslate the lost section, which is why Mosiah begins abruptly with Benjamin already an old king preparing to hand off his throne.
The book also marks a structural transition in Nephite governance, from hereditary kingship to a system of elected judges under Mosiah's reforms, a change the text frames as a response to the dangers of unrighteous kings such as Noah. Alma's founding of a church at the Waters of Mormon, independent of any king's authority, is read within the tradition as an early model for the religious community structure the rest of the Book of Mormon assumes, and Alma himself becomes the founding figure whose descendants narrate much of what follows.
Latter-day Saint editions today print explanatory chapter headings at the start of Mosiah to orient readers around this gap, since the surviving text supplies no in-text recap of what a hypothetical opening section, describing Mosiah's coronation and early reign, might once have covered before it was lost.