The Moonstone (original French title: La Pierre de Lune) is a Johan and Peewit comic adventure which was written and drawn by Peyo. It was first published by Dupuis in 1955 in Spirou magazine, issues 891 to 912, and later in book form.
It was the first Johan adventure to feature elements of magic, turning it into a sword-and-sorcery series. It also marked the first appearances of Homnibus the enchanter and his servant Oliver who would appear in later adventures of Johan and Peewit and the Smurfs.
One evening, at the King's castle, Johan and Peewit see a young man approaching the battlements before falling off his horse. When they go to help him, they find that he is suffering from wounds inflicted by a sword or dagger. They take him back to the castle where he is treated for his injures and left to rest. Later another young man calling himself Lord Boustroux arrives and is invited to stay at the castle.
When the injured man wakes up he tells Johan and Peewit that his name is Oliver and that he is the servant of Homnibus, a charming and peaceful enchanter or wizard. Homnibus sent Oliver to a distant land to acquire a powerful stone. On his way home, he was ambushed by men led by Boustroux who wanted the stone. He managed to escape but was wounded in the process.
Oliver describes Boustroux as an evil man who is not actually a nobleman. When he learns that Boustroux is at the castle, he entrusts the stone to Johan and Peewit and asks them to take it to Homnibus. He then loses consciousness, muttering something about a spell book. Boustroux then enters the room and searches Oliver's clothes; unable to find the stone, he guesses that Johan has it.
That night, Boustroux sneaks into Johan's room, but is confronted by a ghost and a demon who scare him off. The ghost and demon were Johan and Peewit in disguise; they had guessed that Boustroux would try to steal the stone. The next day they set off for Homnibus' home but are ambushed on the way by two men hired by Boustroux. However, in the fight that follows, Johan and Peewit overcome the men and get away.
Johan and Peewit reach the home of Homnibus the enchanter only to see Boustroux running out. He gets away before they can catch him. In the house, they find a man lying unconscious on the floor and assume him to be Homnibus.
When he recovers, Homnibus proves to be a very hot-tempered, rude and unpleasant man who insists on being called "master" and even treats Peewit the dwarf as a little boy. When Johan tells him about the previous events, Homnibus denies knowing anything about Oliver, Boustroux or the stone (which he is supposed to have sent Oliver to acquire) but is quite happy to keep it since it is a very beautiful diamond.
Homnibus then discovers that a valuable spell book has disappeared and angrily accuses Johan and Peewit of the theft. It takes some trouble to calm him down and convince him that Boustroux is the real thief. Johan is intrigued as to how Homnibus does not appear to know anything about Oliver, Boustroux or the stone.
That night, the two men hired by Boustroux break into Homnibus' home is an attempt to steal the stone but are driven away by Johan and Peewit who, assuming that they have the stone, set off in pursuit — not realizing that it is still in Homnibus' possession. They pursue the men to Boustroux's cottage where they are ambushed and taken prisoner. Boustroux then makes a potion which he forces Peewit to drink. The potion puts Peewit under a spell and he sets off to get the stone from Homnibus.
While he is away, Boustroux tells Johan that the moonstone is an essential element in a potion which Homnibus learned about in the spell book Boustroux recently stole. Boustroux even reveals that he was once an aid to Homnibus — who previously told Johan that he did not know Boustroux. The potion will grant amazing powers to whoever consumes it and Boustroux intends to use it to become a feared and powerful ruler.
Peewit steals the moonstone from Homnibus and takes it to Boustroux who proceeds to make the potion. But while he is occupied with the preparation, Peewit, now normal again, frees Johan who attacks Boustroux. In the fight that follows, Boustroux's cottage catches fire and they are all forced to flee.
Leaving Peewit behind, Johan goes in pursuit of Boustroux. However, they fight in the presence of two peasants who assume that Boustroux is a nobleman being attacked by a robber and capture Johan. Boustroux is about to beat Johan with a stick when Peewit arrives accompanied by Johan's friend Count Tremaine. Boustroux flees across a river but is caught in a whirlpool and drowns.
Tremaine tells Johan and Peewit about how he went to the King's castle and met Oliver who told him about the moonstone. Together they went to see Homnibus where they learnt about the theft of the spell book and the moonstone. Tremaine set off to look for Boustroux and met Peewit.
When they return to Homnibus' home, they find him strangling Oliver and accusing him of stealing the spell book. When he sees Peewit, Homnibus angrily demands the return of the moonstone and chases after him. Peewit knocks him out with a heavy piece of wood.
When Homnibus recovers he is a lot calmer and nicer than he has been so far. He recognises Oliver but has no idea who Johan and Peewit are! The last thing he remembers is fighting with Boustroux when the latter stole the spell book the previous day. Johan concludes that Boustroux knocked him out in such a way that he lost his memory.
More peaceful and relaxed, Homnibus even sees the loss of the spell book and the moonstone as for the best given what Boustroux intended to do with them. He even thanks Peewit for knocking him on the head and restoring his memory.
Boustroux's body is later found. The spell book is returned to Homnibus and he gives the moonstone to Johan in order that it can be put somewhere safe, away from the spell book, and not be used for evil.