Sefer Yonah (Jonah) tells the story of Yonah ben Amitai, a navi (prophet) whom Hashem sends to warn Nineveh, the great city of Ashur (Assyria). It is one of the Trei Asar (the Twelve), the twelve short books of the Neviim Acharonim (the Later Prophets), which the Gemara (Bava Batra 14b) says are written together as one sefer. Its four perakim are told as a story rather than as a series of prophecies. (Melachim Bet 14:25 names Yonah ben Amitai as the navi who foretold that King Yeravam ben Yoash would restore the borders of Yisrael.)
Hashem tells Yonah to go to Nineveh and call out against its wickedness. Instead Yonah runs away toward Tarshish, a distant place across the sea, on a ship from Yafo (Jaffa, on the coast of Eretz Yisrael). Hashem sends a great storm. The sailors cast lots, the lot falls on Yonah, and at his own word they throw him into the sea, which then grows calm (1). Hashem sends a great fish that swallows Yonah. After three days and three nights inside it, Yonah davens to Hashem, and the fish spits him out onto dry land (2).
Hashem sends Yonah to Nineveh a second time, and he warns that in forty days the city will be overturned. The people of Nineveh, from the king down, fast, put on sackcloth and turn away from their evil ways and their violence, and Hashem does not bring the disaster (3). Yonah is angry, because he knew from the start that Hashem would have mercy. Hashem grows a kikayon (a castor plant - Shabbat 21a) to shade him and then makes it wither. He tells Yonah that if Yonah cares about a plant, Hashem cares far more about a great city full of people and animals (4).
The whole of Sefer Yonah is read as the haftarah at Mincha of Yom Kippur, followed by Michah 7:18–20.