God instructs Jonah to go to Ninevah and warn them of their impending destruction if they don't repent of their evil ways.

Refusing to help save the hated Assyrians, Jonah flees.
Once at sea, a great storm threatens to capsize the boat. Knowing he's to blame, Jonah begs to be thrown overboard. Immediately, Jonah is swallowed by a "great fish" which, after three days, regurgitates him safely on dry land. Jonah does God's bidding, the Ninevites reform and are spared. It is this tale of repentance that we read during Mincha on yom Kippur - The day of Atonement.