Castle Glower isn’t an ordinary castle. On Tuesdays, the castle rearranges itself, adding a new room, staircase, secret passage or even a wing. The castle is very opinionated and even responsible for choosing who becomes king and queen.
King Glower, the 79th sovereign of the land of Sleyne, married the daughter of the royal wizard after the castle guided them into the same room and sealed the doors for a day. The king’s oldest son, Bran, kept finding his rooms full of books and magical equipment. His second son’s (Rolf’s) bedroom was moved closer to the throne room. King Glower sent Bran to wizardry college and declared Rolf his heir.
Eleven-year-old Princess Celie loves the castle, and the castle loves her. Celie knows the castle best and makes an atlas of it, always carrying pencils and paper to sketch anything new. On the same Tuesday that the king and queen travel to the College of Wizardry for Bran’s graduation, Celie discovers a new tower with magical spyglasses that allows her to see farther than is possible. The royal couple leaves Celie with Rolf and their older sister, Lilah.
Two weeks after the king and queen leave, the royal carriage returns filthy, battered and studded with arrows. It carries only one bloodied soldier, who announces that the king, queen and Bran have been ambushed and killed. The soldier describes the ambush, but admits that he did not see bodies of the royal family before he escaped. Fourteen-year-old Prince Rolf sends out a search party to find them. From the moment the soldier arrived bearing his news, the Sleyne emissary to foreign lands steps in and begins to undermine Rolf’s authority and belittle his decisions.
Despite the ongoing search, a memorial service is planned. Foreign dignitaries, including Prince Khelsh from Vhervhine and Prince Lulath from Grath, attend and stay as guests in the palace. The princes arrive with many soldiers. The royal siblings suspect the princes are looking for opportunities to take the castle and the crown by force. The search party finds the queen’s traveling clothes and wedding ring. They also find bodies that can’t be positively identified, but are suspected to belong to the king and Bran.
Despite everyone thinking that the king is dead, the castle hasn’t changed Rolf’s bedroom to make it more regal or stately as would befit a new king. The king and queen’s bedroom hasn’t changed either. Celie believes this means that their parents are still alive, and the castle knows it. Rolf sends his friend Pogue to the College of Wizardry to ask the magicians to help them track their parents and Bran.
After the memorial service, the foreign princes and their retinues linger on at the palace. Celie overhears Prince Khelsh and the emissary to foreign lands discussing the assassination of Rolf. The royal councilors tell Rolf that he must be coronated as soon as possible, and they will act as regents for the next 10 years, until Rolf is 24-years-old.
The royal siblings object, as they believe their parents are still alive, but Rolf is outvoted by the council and forced to rule under a regency. Later, the castle creates a magic robe that silences Celie’s movements. The castle also creates a passageway leading to the council’s privy chamber and a hole in the wall so she can eavesdrop. There, the councilors sign an agreement making Prince Khelsh the 14th member of the royal council and regent to Rolf. Khelsh and the emissary discuss another agreement that they will force Rolf to sign, making Khelsh the heir when Rolf dies.
Celie, Lilah and Rolf agree to hold all their discussions in the spyglass tower, since only they know about the room and the castle seals the door after they enter. Lilah advises Rolf to be calm and not act surprised when told about Khelsh being on the council. The royal councilors are waiting for Rolf to be angry and behave childishly, to justify the need for regents.
At the coronation, Rolf is calm and dignified, even when the emissary unexpectedly announces to the public that Khelsh is now a member of the council. The next day, an exhausted Pogue rides back to the castle with great news. While the magicians from the College of Wizardry cannot locate the king, queen and Bran, they confirm that the royals are not dead. The siblings are overjoyed. When they tell the council, the emissary and Khelsh threaten to lock Lilah and Celie in a hostage tower unless Rolf does their bidding.
Lilah and Celie run, with the castle providing doors and hallways for them to escape to the spyglass tower. Pogue is arrested, and Rolf is locked in his room. The castle provides Rolf with a passage to the spyglass tower and releases Pogue, who gathers up soldiers to search for the missing royal family.
Additionally, word has spread throughout the castle and beyond that the king and queen are still alive. Celie puts on her magic cloak and spies on the council’s privy chamber where she hears Khelsh say that he is sure the royal family is dead because he sent his best assassins to kill them. Rolf joins his sisters and tells them that if he doesn’t do what Khelsh and the emissary want, they will kill him and put Khelsh on the throne.
Celie sneaks out of the tower to find the other visiting prince in the castle, Prince Lulath. At first, Celie isn’t sure she can trust him, even after Lulath offers to help her. Since the castle has been increasing the size and luxury of Lulath’s rooms, she accepts his help and tells him what Khelsh has been doing.
Lulath tells Celie that Khelsh is so bad that his father has exiled him from his own country. Khelsh paid the emissary to pretend he is in Sleyne by royal decree, but the Vhervhish don’t know he is there. Lulath agrees to send letters to both Grath and Vhervhish, telling the respective kings about Khelsh’s actions.
In the tower, the magical spyglasses show Rolf in the throne room. The council is trying to bully him into sighing paperwork appointing Khelsh as his heir. Rolf knows that Khelsh will have him killed as soon as he signs, so he stalls for more time.
The siblings, with the help of the maids and the castle itself, play pranks on Khelsh and the councilors. They put holes in their clothes, put manure on the bottom of their shoes, remove their chamber pots and the castle opens all the windows to their rooms when it rains. The castle rearranges the councilors rooms to a dark, dank hallway, very far away from the throne room. Khelsh retaliates. He casts a spell, and the castle grows still and cold.
Celie is horrified that her beloved castle has been killed. This also means that she and Lilah are trapped in the spyglass tower. They find rope, and Lilah lowers Celie down to a roof where she works her way down to the ground. Celie finds the staff and tells them all to leave. Celie wants them to unite the citizens of Sleyne and come back and lay siege to the castle.
Using her extensive knowledge of the castle’s layout, she tells the staff how to escape through hidden passageways. Celie finds Rolf and Lulath arguing with Khelsh and the emissary. The emissary tries to grab Celie, but she escapes and takes Rolf and Lulath with her. Rolf and Lulath escape the castle, intending to raise an army, while Celie goes back to get Lilah out of the spyglass tower.
Both girls get out of the tower, but on the way out of the castle, they find two of the traitorous councilors who are now remorseful for helping Khelsh. They tell the girls that the castle has not been killed; it is just sleeping. As the girls move through secret passageways, they pass their parents’ bedroom.
Celie sees the crown. Celie goes back for it, not wanting to leave the crown for Khelsh. She finds Khelsh putting the crown on his head. Celie yells and throws a cold torch at him, startling the prince into dropping the crown, which Celie retrieves and runs away with. Soon she is cornered and forced to run up to the guard tower. From the top, she sees armies camped outside the castle, including those of Vhervhish and Grath. In front of the armies of Sleyne are her parents and Bran.
Khelsh reaches Celie and demands the crown. She refuses and throws it into the moat below. The emissary takes Celie as a hostage, demanding that all the armies leave or he will kill her. Celie reaches out and touches the stones of the castle, telling it that it is still alive, and she feels a warmth and a rumble.
She headbutts the emissary, and in her fight with Khelsh, he cuts her arm with a knife. She bleeds onto the castle stones. Celie removes her stuffed toy lion, Rufus, from her dress and drops it onto the stones. Rufus transforms into a griffin. Celie stumbles away and falls off a castle tower to the courtyard below.
Celie wakes up to find that the castle is awake and has saved her. It turned the hard stones she fell on into soft stones. The griffin took Khelsh and disappeared. Celie is overjoyed to see her parents and brother. The queen explains that during the ambush, Bran used magic to protect them, but he and the king were still badly hurt. They hid in a shepherd’s cottage while the king recovered, all the while hiding from Khelsh’s assassins, who attacked again when they decided to come back to the castle. Pogue and the soldiers from Sleyne saved them.
Vhervhine and Grath allied with Sleyne to fight against Khelsh. After so much excitement, Celie’s arm is bandaged, and her family carries her into her room. She falls asleep after telling the castle how much she missed it. Castle Glower closes her curtains and paints her ceiling dark with twinkling stars, like the night sky.