Spoiler alert: this chapter is where the Israelites screw up big time, and Moses has to bail them out. Here's how it all goes down.
While Moses was chatting up God on the mountain for forty days, the people were getting antsy. Moses did just sort of wander off into the desert, claiming he knew where he was going. What gives?
So the people go to Aaron and tell him to give them gods to worship.
Aaron, without batting an eyelid, tells the Israelites to bring him their gold, and he casts a giant golden calf out of their trinkets. Impressive.
Aaron then flips the party switch, and everyone rejoices around the golden calf.
God is obviously not happy—remember the first commandment? He tells Moses to hang out for a few minutes while he destroys all the Israelites. Afterward, God promises, he will make a great nation of Moses (sound familiar?).
Moses convinces God to chill out, reminding him that they just did all this work to get these people out of Egypt.
Then he takes the tablets written by God down the mountain. Joshua hears the party noises, and wonders if the camp is under attack. Moses tells him that these people are as good as doomed.
Moses reaches the camp, sees everyone rejoicing around, and smashes the tablets in anger. Then he burns the calf, grinds it up into powder, mixes the gold-dust with water, and forces the Israelites to drink it.
Moses yells at Aaron, who blames the people; he says, "I threw [the people's gold] into the fire, and out came this calf!" (32:24).
Moses draws a line in the sand, and the Levites (a tribe of Israel) come over to Moses. He tells them to kill three people each. Yep. About 3,000 Israelites are slaughtered by the Levites. Didn't it say somewhere that murder was bad?
Moses pleads the people's case with God, and God sends a plague down on the Israelites for their disobedience.