Genesis 18 is the account of Abraham's encounter with three visitors at the oaks of Mamre and God's revelation about the coming judgment on Sodom.
The chapter begins with the LORD appearing to Abraham by the oaks of Mamre while he was sitting at the entrance of his tent during the heat of the day. Abraham saw three men approaching, rushed to meet them, and offered generous hospitality. He had Sarah prepare bread while he selected a choice calf to be prepared, and he served them under a tree.
During the meal, the visitors asked about Sarah's whereabouts and then announced that, when they return in a year, Sarah would have a son. Sarah, listening from the tent entrance, laughed to herself in disbelief because both she and Abraham were elderly. The LORD questioned why Sarah laughed and reminded them that nothing is too difficult for the LORD. Sarah denied laughing (out of fear), but the LORD confirmed she did just that.
As the men prepared to leave and go toward Sodom, the LORD decided to reveal His plans to Abraham. The LORD told Abraham about the outcry against Sodom and Gomorrah due to their grave sin and said that He would go investigate the situation.
This led to a remarkable negotiation between Abraham and the LORD. Abraham approached and asked if the LORD would destroy the righteous with the wicked. He began by asking if the LORD would spare the city if 50 righteous people were found there. God agreed, and Abraham progressively lowered the number — from 50 to 45, to 40, to 30, to 20, and finally to 10. Each time, the LORD agreed to spare the city if that number of righteous people could be found.
Genesis 18 ends with the LORD leaving after this conversation and Abraham returning to his place.
