All right, so, God looks around and sees that pretty much everyone sucks,

all except this Noah dude. God figures, "Peh, I'm starting this thing over from scratch. You, Noah--you're the new Adam for the soggy postapocalyptic world. So, build this enormous boat thing to live in for a while. Oh, and while you're at it, can you get some animal-kingdom progenitors to hop aboard with you? Just two of each living thing, that's all. What? Dude, you're going to be surrounded by water, there will be places to put all the poop and pee. Nah, really, don't worry about the smells--I'll take care of all that. No, seriously!"

So Noah conscripts his family and gets to work on building a huge boat. I guess they didn't have much call for boats back then, or at least big ones, because his neighbors were all like, "You're nuts."

Pretty soon it starts raining, and Noah and his family and his wild kingdom board the ark. Or maybe they went in before the rain started, I've always been fuzzy on that detail.

It keeps raining and raining, to the point where all the land is covered. IOW, everything's dead except what's floating in the boat. Meanwhile, they're all getting REALLY sick of each other.

Eventually, it seems that the rain has stopped, so Noah sends a dove out a few times, and on one trip it comes back with an olive branch, which is assumed to signal that the water has gone down enough that at least one olive tree is visible. How it survived a catastrophic flood, I'm not sure. But we're talking about a whole bunch of animals and people living in a boat for a month, so I'm not gonna lose the forest for the trees here.

Then they get out and there's a rainbow.

The end!