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One-sentence summary: Aaron's sons burn incense to the Lord that He has not commanded them, and the Lord calls this "profane fire," and fire comes down from the Lord and devours them.
The Lord tells Moses to take Aaron and his sons and their garments, along with some anointing oil, a bull as a sin offering, two rams, and a basket of unleavened bread, and to gather the whole congregation of Israel at the door of the tent of meeting. Moses does as God commands, then he washes Aaron and his sons with water and girds them with the priestly garments before all the people. He also anoints the tabernacle and all that is in it, sprinkling some on the altar seven times, anointing the altar and all its utensils. He pours some oil on Aaron's head, too, to consecrate him. He brings the bull for the sin offering, and Aaron and his sons lay their hands on the sin offering, and Moses takes the blood and puts some on the altar to make atonement. He takes the fat and burns it on the altar, but the bull's flesh and hide, he burns with fire outside the camp as the Lord commanded. He also does the same with the ram as a burnt offering, and cuts it into pieces, burning all of it, including the fat, but washing the insides and legs with water. He brings the second ram, the ram of consecration, and Aaron and his sons lay their hands on it, and Moses kills it. And Moses takes some of the blood and puts it on the tip of Aaron's right ear, thumb and big toe (the right ones). He then brings Aaron's sons and does the same to them. He sprinkles the blood around the altar and takes all the fat and the right thigh, along with one unleavened cake with oil and a wafer and gives them to Aaron and his sons as a wave offering to wave before the Lord. Then, Moses burns them on the altar as a consecration offering. Moses takes the breast and waves it as a wave offering before the Lord, as the Lord had commanded. Then, he takes some of the anointing oil and the blood from the altar and sprinkles them on Aaron and his garments and his sons and theirs. Moses tells them to boil the flesh at the door of the tabernacle of meeting and to eat it there with the bread in the basket of the consecration offerings. Whatever remains, he tells them to burn it with fire, and they must not leave the door of the tabernacle for seven days, until their days of consecration are ended. And so, the Lord will consecrate them for seven days to make atonement for them. And Aaron and his sons obey.
On the 8th day, Moses calls Aaron and his sons and all the elders of Israel and tells Aaron to take for himself a young bull as a sin offering and a young ram without blemish and to offer them before the Lord. He tells him to tell Israel to take a kid of a goat for a sin offering and a calf and a lamb of the first year without blemish as a burnt offering, and also a bull and a ram as peace offerings, and to sacrifice before the Lord, along with a grain offering mixed with oil, because "today, the Lord will appear to [them]." They bring the sacrifices, "and all the congregation [draws] near and [stands] before the Lord." Moses tells them that the glory of the Lord will appear to them and tells Aaron to go to the altar and offer his sin and burnt offering to atone for himself and the people. And Aaron obeys, doing it just the way the Lord commanded. So he offers the sin offering, burnt offering, peace offerings, and grain offering, just as the Lord commanded Moses that they should be offered. Then, Aaron lifts his hand toward the people and blesses them. Moses and Aaron go into the tabernacle of meeting and come out and bless the people, and the glory of the Lord appears to all the people, and fire comes down from before the Lord and consumes the burnt offering. And when all the people see it, they shout and fall on their faces.
Then, Nadab and Abihu, the sons of Aaron, take their censers (for burning insense) and put fire in them, and then put incense on them and "[offer] profane fire before the Lord which He had not commanded them, so fire [goes] out from the Lord and [devours] them, and they [die] before the Lord." Moses says to Aaron that the Lord had commanded him that to whoever comes near Him, He must be regarded as holy, and be glorified before all the people, so "Aaron [holds] his peace." Then, Moses has Aaron's cousins carry the bodies of Aaron's sons from the sanctuary out of the camp. Moses tells Aaron and his other two sons not to publicly mourn for them, "lest they die and wrath come upon all the people," but to let the people instead mourn for the fire the Lord has caused (and how they have displeased Him.) He also says they must not leave the tabernacle. Then, the Lord speaks to Aaron and commands him not to drink any wine or alcohol when he goes into the tabernacle, lest he die, and to keep this statute for all generations, so that he can "distinguish between holy and unholy and between unclean and clean, and so that [he] can teach the children of Israel all the statutes that the Lord has spoken to them by the hand of Moses. Moses speaks to Aaron and his two sons that are left, Eleazar and Ithamar, and tells them to take the grain offerings left of the offerings made by fire and eat them without leaven beside the altar. He also gives them the instructions he gave to Aaron's late sons. However, when Moses looks into the goat of the sin offering and sees it is burned up, he is angry of Eleazar and Ithamar, because they did not eat it in a most holy place, and that "God has given it to [them] to bear the guilt of the congregation, to make atonement for them before the Lord." Its blood was not brought into the holy place, and they did not eat it in a holy place. Aaron says to Moses that they offered their sin offering and burnt offering before the Lord, and misfortune still came upon him, and that if he had eaten the sin offering, he did not think it would have been accepted in the sight of the Lord. And this answer satisfies Moses.
Thoughts/Discussion Questions:
It is wise to simply obey God. We do not have to know why God asks us to do something; we just need to obey, for our own good. Aaron's sons are a lesson to us that "obedience is better than sacrifice." The God of the old testament is the same God today. We need to fear Him and have proper respect, not taking His mercy and grace for granted or treating holy things in an unholy way.