Eric and Gwendolyn Chant are orphans.  Eric's nickname is Cat, given to him by his sister.  Their parents both die, along with many others, in a steamboat accident.  The children were able to survive because Gwendolyn is a witch and she floats.  Cat had never had powers of his own.  They go to live with a woman named Mrs Sharp who takes care of them from a trust fund set up by the townsfolk.  Mrs Sharp starts Gwendolyn on magic lessons using letters with Chrestomanci signatures on them as payment.  Gwendolyn writes Chrestomanci a letter and he later arrives to take the children to live with him.

Once they get to the Castle they are shown to their rooms.  Gwendolyn, from the start, is upset that she does not get to enter from the front doors with a grand announcement.  She feels that she is not getting the proper recognition for her talents.  They settle into their rooms and sleep.  At breakfast they meet Julia and Roger, Chrestomanci's children.  They later meet Milly, his wife.  They go to lessons which are taught by Mr Saunders.  When Gwendolyn learns that she is not to have magic lessons, she rebels.  She performs several magical tricks to get at Chrestomanci including causing ground hogs to tear up the lawns, moving all of the trees up against the castle and summoning apparitions.

When she sends several creatures and apparitions to a dinner party that Chrestomanci is giving, Chrestomanci and Mr Saunders go to her room to punish her.  Mr Saunders spanks her with his shoe and takes away her magic while Chrestomanci boxes Cat's ears for letting her do the things that she did.  The two children go to bed and when Cat awakens, he runs to Gwendolyn's room to find that there is another girl there.  She looks enough like Gwendolyn that most people could not tell the difference except that Cat can.  Her name is Janet and she comes from another world.  They find a note left by Gwendolyn saying that she has fled to another world.  Cat has to teach Janet about things is his world.  Before they go to breakfast, Euphemia, a maid, walks into the room and immediately turns into a frog.  It is a spell left over from Gwendolyn and they do not know what to do to return her back to her normal self.  They put her in the wardrobe until they can figure it out.  They go to breakfast where Janet is picked on by Julia and then they go to lessons.  During lessons, Chrestomanci shows up with the frog.  Cat lies and says that he turned her into the frog to protect Janet.  Mr Saunders cannot turn her back so they get Cat to do it.  It works even though he has no magic ability. 

As Cat and Janet return to the Castle from the yard, they meet Mr Baslam who sold Gwendolyn the dragon's blood.  She paid him with her mothers old earrings and they were charmed to yell out when the wrong person had them.  He demands 50 pounds for the dragon's blood and wants 20 pounds by Wednesday.  Euphemia's boyfriend shows up and challenges Cat to a duel because of what he did to her.  On Sunday at 3:00 he will show up as a lion and he expects Cat to show as something to fight him, if not he has set a ball of clay in Cat's room that will turn him into a frog.

Janet gets a letter from Mr Nostrum, he old magic teacher and they are to meet him in the village.  They find out that he wants Cat to go to the forbidden garden by the Castle at 2:30 on Sunday so that he can unlock the gates to other worlds that Chrestomanci has kept guarded.  How he can do this is not explained but he and Janet decide that they might need to stay way from there.  Instead of facing all of the problems, they decide that they will run away and go back to see Mrs Sharp.  A car approaches and Julia, Roger and Milly get out and take them back to the Castle.  That night, Janet discovers that Cat has powers and that he has nine lives.  She remembers seeing the matchbook in Gwendolyn's box and tells Cat about this.  Cat can't believe it and so he strikes a match.  He instantly bursts into flame and Janet calls for Chrestomanci.  He appears and helps to douse Cat.  He then explains that he does indeed have nine lives and that only very powerful enchanters do.  It has been Cat all along that has the powers and Gwendolyn has just been using his powers for herself.

Cat and Janet have to stay behind Sunday because of the problems Gwendolyn had caused the last Sunday.  They don't mind as they are going to go to the garden anyway and try to use dragon's blood to get Janet back to her home world.  They get the dragon's blood from Mr Saunder's workshop and head out to the garden.  They have some trouble getting to the garden because it seems to slip by them as they walk.  They have to avert their eyes in order to get there.  They finally arrive at the center of the garden where there is an old apple tree and a broken archway.  Cat sprinkles some dragon's blood on the arch and they see Gwendolyn being treated as a queen.  She sees them and somehow closes the gateway.  Suddenly several warlocks and witches appear.  Gwendolyn's teacher, Mr Nostrum, used a postcard with Cat's signature on it to arrive in the garden once he got to its center.  He then calls Chrestomanci who, upon arrival, is quickly bound by silver.  Silver is his Achilles heal and he cannot do magic with it on him.  Gwendolyn returns as Janet is sent back to her world.  She tells the wizards about Cat's nine lives.  Evidently, they were going to kill him in order to open the ways to the other worlds.  She tells them that they will have to kill him several times in order to do this. 

Chrestomanci tells Cat how to free him from the silver chains and then Chrestomanci calls for help.  When help arrives they are still outnumbered even though all of the people from the Castle are there except for Milly.  Cat realizes that Gwendolyn is using his powers to block her summoning and takes control back.  Milly arrives finally and Chrestomanci and friends round up all of the witches and wizards.  They take their powers away and return them to where they came.  Gwendolyn jumps through the gateway into the world where she was queen and Cat seals her off for good.  Janet returns and Chrestomanci explains that all of the alternate versions of Gwendolyn were better off in their new worlds except for Janet.  Janet says she likes it better here and that they won't have to send her back.  Cat is still sad that his own sister could have turned against him like she did.

Gwendolyn gave vent to her fury in her room after dinner.  She jumped on her bed and threw cushions about, screaming.  Cat stood prudently back against the wall waiting for her to finish.  But Gwendolyn did not finish until she had pledged herself to a campaign against Chrestomanci.

"I hate this place!" she bawled.  "They try to cover everything up in soft, sweet niceness.  I hate it, I hate it!"  Her voice was muffled among the velvets of her room and swallowed up in the prevailing softness of the Castle.  "Do you hear it?" Gwendolyn screamed.  "It's an eiderdown of hideous niceness!  I wreck their lawn, so they gave me tea.  I conjure up a lovely apparition, and they have the curtains drawn.  Frazier, would you draw the curtains, please!  Ugh!  Chrestomanci makes me sick!"

"I didn't think it was a lovely apparition," Cat said, shivering.