His discourse in chapters 2 through 9 is considered by many Book of Mormon readers to be a significant piece of the Book of Mormon. He speaks of his principles of leadership, he emphasizes that he has worked himself as to not burden his people with taxes, and that the laws he has enacted are based on the laws of God. He states that serving one another is equivalent to serving God. He speaks of an angelic visitation and prophecies of Jesus Christ, his birth, identifying his mother as being named Mary, his ministry and miracles, his suffering, death and resurrection. He speaks of Jesus as being the judge, of his atonement as the means to overcome sin and the tendencies of the natural man in order to become a holy person. He emphasizes the importance to have faith in Jesus and to repent in order to become a child of Jesus Christ through His atonement.
He also decrees that his son Mosiah is the new king. (He is the second king Mosiah, as his grandfather was also King Mosiah.)
Zeniff, whose original mission was to spy on the Lamanites, began to like the land, and decided to try to reclaim it. This led to a conflict in his party which ended in bloodshed, but ultimately he got his way. He made a deal with the king of the Lamanites to have a piece of the land of Nephi. He becomes king of this Nephite colony. They had some altercations with the Lamanites, but prevailed at that time.
Zeniff dies and passes rule to his son Noah. Noah is a wicked king. He is one of the more favorite villains among Book of Mormon readers. He collects exorbitant taxes from his people to build a palace and he and his ministers live a life of comfort, ease and self indulgence. His wicked ways lead the whole colony into wickedness.
King Noah and his priests are angered by this and sentence him to death by fire.
One of King Noah's priests named Alma is stirred by Abinadi's words and pleas on his behalf. He too is accused and he flees. Alma hides and writes down the words of Abinadi. After a period of sore repentance, Alma begins to preach the words of Abinadi and the doctrine of Christ to the people in secret. He gains a sizable following and in chapter 18, Alma begins to baptize those who have accepted Christ. The Lord tells Alma that King Noah has discovered them and will be coming after them. He and his followers flee the land.
The Lamanites attack King Noah and his people and they begin to run. King Noah tells his priests and others to leave their wives and children so they can escape from the Lamanites. Those that follow this command are later angered at themselves and King Noah for leaving their families. The group sentences King Noah to death by fire. They then turn on the priests of Noah who flee before the people, later becoming the Amulonites. King Noah's son Limhi rules, but becomes a tributary monarch to the king of the Lamanites.
Also, King Mosiah by the aide of God translates a set of records which were found by Limhi's people. They tell of a people commonly called the Jaredites. A portion of the record was inserted in the Book of Mormon as the Book of Ether. King Mosiah's grandfather Mosiah, had also translated some writings found on a large stone which touched upon these people.
Among those who don't believe are a son of Alma who also shares the name Alma (but he is usually differentiated as "Alma the Younger"), and King's Mosiah's own sons. One day while they are out and about doing their destructive work, an angel comes to them and tells them to no longer seek to destroy the church. This sight causes them great fear and Alma the Younger faints. He is in an unconscious state for two days and two nights and his father prays for him. When he comes to, he speaks of having waded through much tribulation and finding redemption through Christ. He speaks in much more detail about this experience in the Book of Alma, chapter 36. The experience causes himself and his associates (King Mosiah's sons) to become converted to the Lord and to build up his church.
Mosiah's sons approach Mosiah and tell him that they want to leave Zarahemla to go to the Lamanites and preach to them. This worries Mosiah, but he asks God who assures him that they will be protected and that they will also do much good there. Their journeys and preaching are described later in the Book of Alma beginning with chapter 17.
At the end of the Book of Mosiah Alma (the elder) and Mosiah both pass away.