The Castellac Ring (L’Anneau des Castellac in French) is a "Johan and Peewit" comic adventure written and drawn by Peyo and was first published by Dupuis in Spirou magazine in 1960. It was then published in book form as the eleventh book instalment of the series.
Johan and Peewit are staying at an inn where the only other guest is Prince Hubert, Duke of Castellac, who has recently escaped from the dungeons of his enemy Lord Hercule of Basse-Fosse who kept him prisoner for over three years. A ransom should have been paid for his release but never was.
Johan and Peewit offer to go to a town and get the Duke a horse and he gives them his ring as collateral. They return the next day with a horse only to find that there has been a fight at the inn and that the Duke is missing. They find the innkeeper down the well and rescue him. He explains that during the night some brigands came to the inn to seize the Duke. He fought back but was overwhelmed and taken. The men then threw the innkeeper down the well in order to get rid of any witnesses.
Johan, Peewit and the innkeeper go to the town of Castellac only to witness a welcoming celebration in honour of the Duke. He himself is riding through town, acclaimed by the people, and accompanied by the leader of the brigands who seized him. The leader is in fact Sir Acelin, an equerry or aide to the Duke, who ruled the duchy while he was a prisoner of Hercule of Basse-Fosse.
In a tavern they overhear a conversation which indicates that the Duke was released by Hercule of Basse-Fosse after the ransom had been paid — yet they know that he actually escaped. They conclude that he is a double impersonating the Duke so that Acelin can continue to rule the duchy. They decide to expose him by asking him what he paid as collateral — his ring. That night they break into the castle and confront the Duke in his chamber. At first he does not recognise them and appears to have a problem remembering things. When they ask him what he gave them, he struggles to remember. Johan and Peewit think that they have proven his fraud, but then he suddenly mentions giving them his ring!
At that moment, Acelin appears and, seeing Johan and Peewit, summons the guard. Johan and Peewit flee. They are chased all over the castle and risk being recaptured when a man called Demetrius hides them in his chamber and advises them to go to the castle of Hercule of Basse-Fosse and get evidence of Acelin's treachery. He then gets them out of the castle through a secret passage.
Johan and Peewit set off by sea to Hercule of Basse-Fosse's castle. They gain entry by buying a harp and posing as minstrels. Hercule turns out to be a large, fierce and bad-tempered man who loses his temper when things don't go his way or when people contradict him. He does, however, take a liking for Peewit whose loud and dreadful singing and harp playing reminds the belligerent Hercule of the sound of battle and the clash of arms!
In conversation, Hercule reveals to Peewit that Acelin paid the ransom on the condition that he keep the Duke prisoner; thus enabling Acelin to remain ruler of the duchy. He even shows Peewit a letter from Acelin proposing the deal. The letter is kept in a locked chest and Peewit resorts to all sorts of tricks and deceptions to get the key, all of which fail or backfire.
Meanwhile Acelin learns that Johan and Peewit are at Hercule's castle and guesses what they are up to — the Duke having told Acelin about how he met them at the inn. Acelin sends some men to Hercule demanding that he hands Johan and Peewit over to them. When the men arrive, Johan and Peewit overhear them revealing that they have been sent by Acelin. Now desperate, they knock Hercule out, bound and gag him, open the chest, find the letter and escape.
Found and released by his men, a furious Hercule sets off in pursuit of the fugitive pair. Johan and Peewit reach the coast, dive into the sea and manage to get on board a fishing boat. In exchange for some money, the fisherman agrees to take them to Castellac but they are caught in a storm and swept overboard. Hercule mourns the loss of Peewit and his singing.
Acelin's men return to Castellac and report the drowning of Johan and Peewit. Demetrius, who sent Johan and Peewit to get proof of Acelin's treachery, knows that he is now the only one who can now save the Duke, but he is a coward and is too afraid of Acelin to dare betray him.
Acelin then learns that the Duke is seriously ill and may not live for long. The Duke's nephew and heir hates Acelin and will quite likely exile him, so Acelin summons the local barons and other vassals to the castle. The Duke, looking very pale and withdrawn, reads out of a statement by which he announces that the three years of captivity have seriously weakened him and he is abdicating in favour of Acelin.
At that moment, however, Johan and Peewit suddenly turn up having survived the shipwreck. They still have Acelin's letter to Hercule and accuse him of plotting against the Duke. At that moment Demetrius suddenly finds the courage to confirm their claims. He is a specialist in potions and reveals that he made a magic potion which deprived the Duke of his free will and brought him under Acelin's control. He also reveals how Acelin tracked down and captured the Duke at an inn after the latter had escaped from Hercule's dungeon. Acelin denies this but then Johan and Peewit bring in the innkeeper whom he tried to kill by pushing down a well.
Exposed, Acelin takes the Duke hostage and flees the castle on horseback, but Johan and Peewit go after them, rescue the Duke and capture Acelin.
Acelin is imprisoned and Demetrius is forgiven. A few days later the Duke is on the path to recovery and Johan and Peewit return him his ring. The Duke says that it will be a reminder for all they did for him.
After they have left, Peewit remarks that they will soon be home and be able to live a peaceful, calm existence until Johan drags him into another adventure.