So God, in all his moodswingy glory, decided to wipe out the entire human race.
Except for this one guy, his wife, his three sons and his three daughters-in-law. So Noah gets told to engage in one of the world’s most unlikely acts of carpentry. He builds an Ark in which to place a breeding pair of every kind animal in the world – which, by the way, would totally not fit in the cubic volume of Ark, unless “cubit” is an ancient hebrew word for “mile” – and apparently successfully places them there.
And then God makes it rain for forty days and forty nights. Fortunately, the flooded Earth has a very low albedo, and all this water eventually evaporates into the vacuum of space, allowing the ludicrously small gene pool we are allegedly all descended from to not suffocate from the vast quantities of water vapour in the air. And there’s a rainbow.
As mentioned in:
And down the rainbow rode the Norse gods, and they looked at Noah for a while, told him “no way are you getting into Valhalla” and then rode back up the rainbow to Asgard.