Told you I'd explain! After years of living in Zarahemla, Alma and King Mosiah ended up being pretty solid friends. Which of course meant their sons ended up being pretty solid friends. Sort of. It never mentions why (this is only an abridgement of a much larger record, after all), but eventually Alma the Younger and the sons of Mosiah left the church. Something interesting my institute teacher has said, you can leave the church, but you can never leave it alone. Basically she explained that those who leave the church in outright anger over something can't stop coming back to tear it down. That's what Alma the Younger (I'll just call him Alma II for now) and his friends were up to. They were constantly trying to pull people away, destroy the church. For the son of a prophet to be trying to destroy the church... that hurts. Alma I was praying constantly for his wayward son, and... well, his prayers were certainly answered. ;)
One day while Alma II and Mosiah's sons were out and about destroying the church, an angel appeared to them. He spoke, and the ground shook. Alma II fell to the ground in terror, and the angel told him to get his sorry butt out of the dirt (okay, so he didn't actually say that, but seriously), both literally and metaphorically. He told him he was there because his poor father had been praying his aching heart out for him, and that if he didn't get his head on straight, he was going straight to hell (random insert: I just find it amusing that most of the time, when an angel appears to someone in the scriptures, he tells them not to be afraid, but in this instance he tells Alma II "you'd darn well better be afraid, punk!).
Alma II was so terrified by the experience that he fell to the earth, couldn't speak, and lost the use of his limbs. The sons of Mosiah had to carry him to his father, who gathered the saints to pray and fast for his son. I can imagine several of them had to wonder why he bothered, but hey, that's neither here nor there. After three days (symbolic much?), Alma II finally woke up, praising God and saying he had been forgiven. From that time forward, he and Mosiah's sons dedicated their lives to preaching the gospel... and this is where the real begins!
Fare thee well, friend!