Moses's sister, Miriam, and his brother, Aaron, get a little jealous that God always seems to be chatting it up with Moses. Plus, Moses married a Midianite woman, so everyone knows he's not the cat's pajamas.
But God knows that Moses is humbler and more awesome than anyone, so he calls Miriam and Aaron into the tabernacle for a pow-wow.
God appears in front of the tabernacle as a pillar of cloud. He explains that he usually communicates with humans through dreams and visions and all that good stuff. But with Moses, the two of them just talk face to face. Miriam and Aaron know this, right? So, why are they bad-mouthing Moses?
God is ticked, so when they leave Miriam gets struck with a creepy skin disease. Aaron freaks out and begs Moses not to punish them for being so stupid or worse… kill them.
Moses asks God to heal Miriam, but he tells Moses that they should just throw her out of camp for seven days so she can be ashamed in peace. Afterwards, everything will be copacetic.
So, they toss Miriam to the curb for a week. Luckily, the people wait for her before moving onto their next stop. Otherwise, that would be quite a punishment.