The Book of Joel narrates the story of the Prophet Joel. The book starts with him telling the people that a locust invasion has wreaked havoc on the land. He asked the priests and people to ask forgiveness from God and prayed.
He then goes on to describe a day in the future when the locusts will attack again. This time, he describes the locusts in a way that seems like God will send them as He would his army. He again tells the people to ask for forgiveness.
The manner of asking for forgiveness that he described was from the heart, which meant to change one’s actions, and not merely asking. He then explains that God is all-loving and forgave His people even after they made the golden calf.
He tells them that God will turn the Day of the Lord from punishment into salvation for the people, that the land will become fruitful, and that He will come and dwell among the people. The message of this book is that people should obey their covenant with God and keep asking for forgiveness.