The prophet announces that the day is coming, burning like a furnace. All the arrogant and all who do wickedness will be like stubble, and the coming day will consume them, leaving neither root nor branch. But for those who fear the Lord's name the sun of righteousness will rise, with healing in its wings. They will go out and leap like calves from the stall (vv. 1-2).
On that day the righteous will tread down the wicked, who will be ashes under the soles of their feet, on the day when the Lord acts (v. 3). Then the Lord commands them to remember the law of his servant Moses, the statutes and rules he commanded at Horeb for all Israel (v. 4).
The book, and with it the Old Testament, ends with a promise. The Lord will send the prophet Elijah before the great and terrible day of the Lord comes. He will turn the hearts of fathers to their children and the hearts of children to their fathers, so that God does not come and strike the land with total destruction (vv. 5-6).