Mr. Collins arrives at the Bennets' house for a week-long visit. His purpose, as he makes abundantly clear, is to find a suitable wife, and he expects one of the Bennet girls will do.
Mrs. Bennet lets him know that Jane is already spoken for (not strictly true), so he sets his sights on Elizabeth.
Mr. Collins proposes to Elizabeth. Mrs. Bennet has told Elizabeth she should say yes, but she can't bring herself to marry a man so odious and dislikeable. It's not that he has any vices or truly sinful traits—he's just really annoying.
Three days later, Mr. Collins proposes to Elizabeth's friend Charlotte.
Mr. Collins and Charlotte marry and move into his parsonage on the estate of Lady Catherine de Bourgh.
When Elizabeth visits, he is full of advice about how she should behave around Lady Catherine.
When Lydia runs away with Wickham and then gets married, he sends a letter explaining his shock that Christian folk like the Bennets would receive their fallen daughter and fallen husband into their household after such a disgrace.
Mr. Collins also writes to Mr. Bennet to explain that Elizabeth should think twice before encouraging Mr. Darcy, since it will make Lady Catherine quite angry. We mustn't upset Lady Catherine one bit!