1-11: God used Haggai as a messenger after the Babylonian exile, while the temple lay in ruins. God rebuked Judah for procrastinating on rebuilding the temple as they built and lived in nice houses for themselves. Because of this, God thwarted their productivity and fulfillment in material things. He countered their agricultural work with natural conditions that destroyed many of their crops.
12-15: The people responded in obedience and began building the Lord's house. Then God spoke through Haggai to tell the people He was with them.
2-9: A month into construction the people were discouraged so God reminded them that He was with them and that the future temple would be greater than the previous temple in glory.
10-19: God used ceremonial laws as an illustration to show how their service to him was unclean because it came from sinful people. In the first question he shows they can't make their service holy because they're guilty of sin. In the second, it shows that the work of their hands is defiled. Yet by God's grace he blessed them with prosperity in the land.
20-23: The book closes with God's promise of justice in the end times. It's not clear to me what God's promise of the exultation to Zerubabbel is about, or what it means.