by J. K. Rowling
On the eve of a visit from the king of the neighbouring country of Pluritania, Dora Dovetail dies of overwork, due to frantically trying to finish King Fred's latest costume, despite being ill. Fred, although embarrassed and feeling guilty, declines to visit the seamstress's family. The Dovetails, especially Daisy Dovetail, Dora's daughter, become bitter and disillusioned with him. This leads to a fight between Daisy and her close friend Bert Beamish, Major Beamish's son, when the former insults Fred. Upon hearing of the fight, Fred resolves to be less self-centred.
When a shepherd from the Marshlands begs the king to rid the country of the Ickabog, Fred leaps at the chance to prove himself, riding to the North immediately. However, due to Fred's impetuousness, an accident occurs in the marshes that results in Major Beamish accidentally getting shot by Flapoon, one of Fred's advisors/friends. Seeing the opportunity to take control of the kingdom and become richer, Spittleworth, Flapoon's ally, pretends that Beamish was killed by the Ickabog, scaring Fred and the rest of the army into belief.
On their return, three soldiers, including Goodfellow, a friend of Beamish, and Herringbone, the Chief Advisor, raise objections to the story. Herringbone is murdered and the three soldiers are imprisoned and given a false trial, ruining their names. A heavy tax is imposed on the country, to pay for an "Ickabog Defence Brigade". King Fred is scared into staying within the capital, to prevent him from seeing the poverty the tax has caused. Spittleworth, the new Chief Advisor, has the Dovetails kidnapped, with Dan being sent to prison, and forced to carve Ickabog feet for faking attacks on dissenters. Daisy is sent away to be murdered, although she ends up in Ma Grunter's orphanage.
Several years pass, with the tax doubling, ostensibly to pay for more soldiers. Bert and his mother, Bertha Beamish, the king's head pastry chef, later guess Spittleworth's plot after Bert discovers a miniature Ickabog foot, the relic of a toy made for him by the now insane Dan Dovetail, identical to that of the supposed Ickabog. When she attempts to reveal the plot to the king, she is kidnapped and placed in the dungeons. Bert meanwhile escapes the city with the help of an incorrupt guard as Major Roach leads soldiers to arrest him. Arriving in the city of Jeroboam, Bert meets Roderick Roach, who tells him that Spittleworth killed Major Roach and imprisoned his family upon the latter's failure to capture Bert. However, before the two can go anywhere, they are captured by Basher John, and taken to the orphanage, where Bert meets Daisy. Meanwhile, Bertha Beamish has had the dungeons transformed into a kitchen, where, with the help of the prisoners, she continues in her old job, while slowly pulling Dan Dovetail back from his insanity.
The four children, Roderick, Bert, Daisy, and Martha (a Marshlander who Daisy had befriended at the orphanage) find out that Ma Grunter's orphanage is due for an inspection. If the inspector found Bert, a highly prized fugitive, they knew he would be recognized. They decide to escape before that happens.
Bert and Roderick steal the keys, and the four hike through the cold to the Marshlands, led by Martha. They plan to meet the soldiers of the Ickabog Defence Brigade there, tell them their stories, and sway them to their side. When they reach the Marshlands, they realise that the Brigade has gone south for the winter. Succumbing to the cold, they fall unconscious. While they are unconscious, the real Ickabog carries them away.
The four teenagers end up inside the Ickabog's cave. The Ickabog cooks them food from an abandoned food wagon. After Daisy talks to it, the Ickabog reveals that from it a new batch of Ickabogs will be "bornded". As the feelings experienced by the dying parent Ickabog influence those of its newborn brood, the Ickabog plans to eat the four during the bornding, to ensure that its children, which would normally only eat mushrooms, will become man eaters, to take revenge on and wipe out the humans, the cause of the near-extinction of the Ickabog race.
In Chouxville, King Fred orders an Ickabog to be stuffed, and a ball to be given to celebrate the war. In the dungeons, the prisoners are arming themselves with kitchen knives and Dan Dovetail's chisels. Spittleworth, distracted with the problem of the ball and faking a stuffed Ickabog, fails to notice.
Meanwhile, Daisy persaudes the Ickabog not to eat them, but to reveal itself to the people and live amongst them, so that the people learn not to fear it. The group marches on Jeroboam, with the Ickabog handing out flowers to soldiers and citizens of the city. After overcoming their initial fear, they rally around the Ickabog, fuelled by resentment at having been lied to. After arming themselves (to protect the monster), they march on Chouxville. However, Basher John, having prospered under Lord Spittleworth's regime and realising the danger to it, steals a horse and rides ahead to warn him. Due to being held up, he arrives only as the Ickabog is approaching Chouxville. He breaks into the king’s apartments to deliver the news to Spittleworth, but is arrested, as Spittleworth refuses to believe him. Two spies deliver the same news minutes later. As Spittleworth prepares to investigate with Flapoon, he is confronted by the prisoners, who have escaped. They escape out the apartment, locking the exit, leaving King Fred to face the angry mob.
As the Ickabog approaches Chouxville, Spittleworth goes to confront it with the Brigade. Despite Spittleworth’s orders to the contrary, when Flapoon sees the Ickabog's belly split, he fires his gun, believing that there are people inside controlling it. Bert jumps into the path of the bullet, determined to save the Ickabog at all costs. It hits his father's medal, saving him. The first Ickaboggle to be Bornded kills Flapoon, due to being Bornded in fear of his gun. However, the second Ickaboggle is Bornded kind, as Daisy is comforting the Ickabog as it dies. Spittleworth and the rest of the Brigade ride away. When Spittleworth reaches his mansion to escape with his gold, he is captured by Bert and Roderick – disguised as Scrumble and Withers, Spittleworth’s butler and groom – who had ridden ahead, freed Lady Eslanda, and bound Scrumble who is wrapped up on the floor.
Spittleworth, King Fred, Ma Grunter, Basher John, Cankerby, Scrumble and others who benefited from Spittleworth’s regime are tried and imprisoned in the palace dungeons. Goodfellow becomes Prime Minister and marries Lady Eslanda, Roderick Roach marries Martha, and Bertha Beamish marries Dan Dovetail. The money from Spittleworth’s hoard is returned, allowing Cornucopia to be prosperous once more. A new city, Ickaby, is founded in the north, producing mushrooms, fish, and wool as its specialities. The first-born Ickabog, though initially savage, is tamed by Fred, and finally released, so that when its Bornding comes, the Ickaboggles are kind. Fred dies shortly after. Cornucopia lives happily ever after.