CLAIM: Critics find it implausible that the entire population of a major Pagan city would repent like this. Is this absurd?
RESPONSE: This objection really tells us more about the critic, than it does about the text. The critical theologian has himself rejected the authority of God’s word, so they believe that everyone else does as well. As Paul writes, “A natural man does not accept the things of the Spirit of God, for they are foolishness to him; and he cannot understand them, because they are spiritually appraised” (1 Cor. 2:14). Yet to the believer, God’s word is “the power of God” (1 Cor. 1:18).
In addition, Assyria was ripe for a prophetic visitation like this—especially at this particular moment in history. During this period in history, Assyria went into a dark age. They had been in an expansive period before this, but under Ashur-Dan III (771-754 BC) the people were feeling pessimistic and worried about the future of the kingdom. This is because “a serious plague had befallen the city in 765 and a total eclipse of the sun had taken place on June 15, 763. Another plague had followed in 759.” Of course, solar eclipses were bad omens to ancient peoples. The people of Assyria may have been on edge about their future as a nation, which could have softened them up for Jonah’s preaching. K.A. Kitchen writes that this was a “time of Assyrian recession when that superstitious kingdom might have listened to a visiting prophet alongside its own practitioners.”
Finally, we need to remember that Jesus believed that Jonah’s preaching was effective. He said, “The men of Nineveh will stand up with this generation at the judgment, and will condemn it because they repented at the preaching of Jonah” (Mt. 12:41). If this was merely a parable (i.e. fictional), why would he say this? For instance, a pastor might make a metaphor out of Darth Vader repenting before he died. And he might make the application that you too should repent. However, he would never conclude the metaphor by saying, “And Darth Vader will be in heaven with you, too!”
 Archer, Gleason. A Survey of Old Testament Introduction (3rd. ed.). Chicago: Moody Press. 1994. 345.
 Kitchen, K.A. On the Reliability of the Old Testament. Grand Rapids, MI: Wm. B. Eerdmans Publishing Co. 2003. 375.
 I am indebted to Wayne Grudem for this illustration.