At the Castle, Amber is having her room redecorated. She has the servants rearrange a tapestry before eventually deciding to have them simply hang it in the other room. After they leave, Amber notices a tile on the wall the tapestry was on. As she inspects it, Amber causes the wall to swing and finds herself in a secret passageway. Amber goes down a flight of stairs where she finds another passage with stairs and an underground river. Suddenly, she hears someone's footsteps and goes to hide. The somebody turns out to be Sofia with Behind the Walls in her hand. They are in the secret passage that leads to the Secret Library. Amber confronts Sofia who is shocked to find her. She insists that she go back, however Amber is adamant on finding out where they are and what Sofia is doing, which Sofia cannot say. Suddenly, Sofia's Amulet glows blue. Sofia tries to hide it from Amber, but it is already too late. When the crystalline boat appears, Amber deduces that it is magical. As she climbs into the boat, Sofia tells Amber that she does not have time to explain as she really has to get somewhere. She again tells Amber to go back, but her big sister instead gets into the boat as well. She refuses to leave until Sofia tells her everything. Unable to talk her out of it, Sofia reluctantly explains her role as the Storykeeper during the boat ride to the Secret Library.
Upon entering the Library, Sofia and Amber sit down to receive Sofia's latest story. The Library selects a story called The Tale of the Crystalmasters. The book goes inside the pendulum and the narrator tells them the story about the legendary Mystic Isles, a place made up of countless floating islands homed to different magical creatures and objects. As the story goes, the Crystalmasters on the Isle of Crystals have peacefully grown magical crystals for wizards everywhere to use in their scepters, wands, and amulets for centuries. However, one Crystalmaster has a wicked plan to grow magic crystals on the other Mystic Isles, which could drain all the magic from the creatures who live there forever. When Amber asks where the rest of the story is, Sofia again reminds her of her role as the Storykeeper before telling her she has to go to the Mystic Isles and give this story a happy ending. Amber wants to come along as she wants a magical amulet of her own so she can have her own special adventures. Sofia tells Amber she cannot come by reminding her that she is the Storykeeper and what she is out to do will be dangerous. Amber appeals to her little sister's sense of fairness which makes Sofia concede. Suddenly, the magical elevator drops down and the two girls ride it to the Mystic Isles. The elevator drops them off at a giant dandelion patch where they meet a girl named Prisma, who used to be a Crystalmaster until her sister Azurine took her powers away. Amber deduces that Azurine is the evil Crystalmaster they need to stop, and Prisma tells them they can get to the Mystic Isles if Sofia uses her Amulet to summon unicorns. Sofia does so and three unicorns come down to take them there. Amber is shocked that the unicorns can talk, to which Prisma reveals that all the animals from the Mystic Isles are able to talk. The unicorns fly the trio to the Mystic Isles, and drop them off on the Isle of Unicorns. Upon arrival, they meet a newly-winged unicorn named Skye, who gets stuck in a tree after failing to teach himself how to fly. After Sofia gets him out, he pledges to help her with her quest before giving Sofia and Amber a tour of the Mystic Isles.
The group arrives at the Isle of Dancing Deserts, which is the last Isle they need to cross to get to the Isle of Crystals. During the trek, Prisma and Skye reveal that the Dancing Deserts are so named because they reveal what one desires. Hearing this, Amber asks the desert to show her a magical jewel. It gives her an orange crystal that Amber fails to get powers from. Amber is so annoyed by this that, when they get to the bridge they need to cross, Amber strikes the bridge with the jewel three times. This causes the bridge to collapse in a way that makes Sofia fall off. She gets saved by a man with wings who then saves the others. Prisma raises her hood and looks at the man in fear. The man introduces himself as Orion and tells them that he is a Wind Walker and one of the Protectors, the group who protects everyone on the Mystic Isles from harm. Orion then tells Amber that her magical jewel is a Shatter Stone, a magical rock that destroys anything it gets struck three times against. Hearing this, Amber puts the Stone back. Orion informs them that he is looking for the evil Crystalmaster to which Sofia reveals she is the Storykeeper and is going to the Isle of Crystals to stop her. Orion informs her the villainess was not there when he looked and urges her to keep an eye out. Sofia is confused until Prisma dismisses Orion's claim by saying that her sister is cunning and thus could've fooled Orion and insists that they continue onward to the Isle of Crystals. With the bridge gone, they now have to go through the Isle of Rompkins. Once there, the get captured by the Rompkins, who trick Amber into putting a crown on her head and becoming their Queen. To get away, Sofia and Amber get them to let them go by teaching them manners. The Rompkins release them and wish them luck.
The next day, the group finally arrive on the Isle of Crystals. Prisma points out Azurine and the tower where she locked up Prisma's Terra Crystal, the magical tool Crystalmasters use to perform their magic. After Skye fails to fly yet again, Sofia uses her Animal Transformation power to turn herself into a Crystal Bird. She flies into the tower and succeeds in getting the Terra Crystal, but gets trapped in a net. Azurine sees and goes to her aid. Prisma convinces Amber that Sofia is in danger, and promises to give her an amulet as special as Sofia's if she gets her Terra Crystal. Meanwhile, Azurine's kind treatment makes Sofia realize that Azurine is not what Prisma said she is and makes her suspect that Prisma is not what she seems. Sofia tries to warn her big sister, but Amber, blinded by her own envy and greed, ignores her little sister and gives Prisma her Terra Crystal. Prisma uses the Crystal's magic to return to her true form. Revealing herself to be the real evil Crystalmaster, Prisma declares that the magic of the Mystic Isles will be all hers. Azurine arrives and Prisma uses her magic to trap her inside a crystalline cage, Horrified, Sofia asks Prisma why she's doing this and Prisma explains how she always felt overshadowed by her sister and wants to be the greatest Crystalmaster ever, even if that means harming the other inhabitants of the Mystic Isles.
Prisma gives Amber an amulet before sending her, Sofia, and Skye plunging down a Crystal Vortex. Sofia uses her Animal Transformation power to turn herself into a Flying Unicorn and helps Skye fly down. Amber tries to follow suit only to discover that her amulet is not magical. When they land safely, Amber voices how terrible things are. Sofia agrees stating that Prisma has to be stopped, only for Amber to reveal she is just upset that her amulet is not magical. Disgusted, Sofia angrily reminds her that it was her desire that caused this situation. Amber retorts that she made an honest mistake due to thinking Prisma was their friend, to which Sofia snaps that she tried to warn her not to give Prisma her Crystal. Skye breaks up the argument and reminds them of the mission. Sofia agrees and is determined to stop Prisma but Amber just wants to go home. Sofia tells her big sister to stop thinking of herself as everyone in the Mystic Isles need help, to which Amber states that there is still the Protectors. The group set off to find Orion. Meanwhile, Prisma uses her her magic to cover the Mystic Isles with crystals starting with the Isle of Unicorns, which the group discovers when Skye loses his wings and horn. Orion appears but he loses his wings. Harrumph the Rompkin saves him and the group tell him what is going on. Orion informs them that if they destroy her Terra Crystal all the damage will be undone. Sofia offers to help but Orion tells her to leave it to the Protectors. Amber is only too happy to seize upon the excuse to go home. However, when Prisma shrinks Harrumph and covers the Isle they are on in crystals, Sofia insists that they stop Prisma anyway. Prisma is amused that Sofia actually wants to fight her. Azurine warns her that Sofia can stop her but Prisma retorts that is impossible, as all Sofia has by her side is a unicorn that cannot fly, a shrunken Rompkin, and a big sister who is a spoiled, self-serving brat who only thinks about herself. Meanwhile, Amber tries to talk Sofia into going back but Sofia insists on the ground that it is the right thing to do. Amber voices her fear of not feeling up to it only for Sofia to state she is, by reminding of times she has saved the day and together they set off to stop Prisma.
To destroy Prisma's Terra Crystal, the group set out to obtain a Shattering Stone. Prisma tries to stop them by covering the Isle of Dancing Deserts in Crystals but the group keeps going, since the Stone's magic is not drained due to being made on another Isle. Since she cannot remember which Isle that was, Prisma traps them inside crystal cages. Sofia gets out of hers by using her Shrinking ability and, upon obtaining a Shattering Stone, frees the others. When they arrive at Prisma's home, Amber distracts Prisma by making her give her amulet powers, such as the ability to conjure mirrors, while Sofia frees Azurine with the Stone. Azurine and Prisma fight and Prisma only wins when she makes it so Azurine cannot use her wand. Sofia charges at Prisma and hits her Terra Crystal twice with the Stone before getting trapped too. While Harrumph retrieves the Stone, Prisma tries to zap Amber, who uses her Mirror Conjuring ability to reflect her attack and trap her before taking the Stone from Harrumph. Prisma tries to dissuade Amber from using the Stone by reminding her she will lose her powers. Amber uses the Stone to destroy her Terra Crystal anyway, the Crystals are gone, and Prisma's powers are destroyed. Prisma runs for it and Sofia and Skye give chase, during which Skye finally becomes able to fly. The pair nab Prisma just as the Protectors arrive to arrest the Crystalmaster. The Protectors are impressed with Sofia and recruit her to become a Protector of the Ever Realm. After Sofia accepts, The Protectors tell her they will send word of when her training will begin before ordering her to return to Enchancia. Sofia picks up Amber and the two sisters go home with Skye on the scenic route.
- Ariel Winter as Princess Sofia
- Darcy Rose Byrnes as Princess Amber
- Andrew Rannells as Skye
- Ming-Na Wen as Vega
- Megan Hilty as Prisma
- Colin Salmon as Orion
- Rachael MacFarlane as Azurine
- Hugh Bonneville as Narrator
- Fred Tatasciore as Harrumph
- Fritz Sperberg as Grump
- Brian Cummings as Bash
- This is the fifth TV Special movie in the Sofia the First series, following Once Upon a Princess, The Floating Palace, The Curse of Princess Ivy and Elena and the Secret of Avalor.
- Unlike the previous specials, this one premiered in the Summer and on a Saturday.
- This is the sixth installment of the Secret Library story arc, preceded by "The Secret Library", "Olaf and the Tale of Miss Nettle", "The Tale of the Noble Knight", Elena and the Secret of Avalor and "The Secret Library: Tale of the Eternal Torch".
- This is the third time Sofia has someone coming with her on her Storykeeper adventure, first was Aunt Tilly in "The Secret Library" and the second was James in "The Secret Library: Tale of the Eternal Torch".
- The Mystic Isles was mentioned in "Day of the Sorcerers", foreshadowing this special.
- This special is the second time Andrew Rannells and Megan Hilty voice characters on the show. The first time was in "Beauty is the Beast", where Rannells voiced Morris the Goblin and Hilty voiced Princess Charlotte.
- This is the fifth time Sofia has a ponytail.
- This is the third Secret Library episode to have a plot twist, the first being "Olaf and the Tale of Miss Nettle" and the second being "The Tale of the Noble Knight".
- Moral: The needs of others must come before your own.
- Amber discovers another secret passageway in the castle that leads to The Secret Library.
- This is the first special in the series where Roland, Miranda, and James do not appear.
- When Amber felt like a downer, Sofia reminded her all of the right things she did from past episodes like "Princesses to the Rescue!", "Her Royal Spyness" and "Sidekick Clio".
- It is revealed that the world Sofia lives in is called the Ever Realm.
- Unlike Once Upon a Princess, The Floating Palace, The Curse of Princess Ivy and Elena and the Secret of Avalor, this is the first special not to feature a special title card.
- This is the second time Amber finds out about Sofia's amulet being magical and desires a magical amulet of her own, the first being The Curse of Princess Ivy.
- Unlike in The Curse of Princess Ivy, Amber does not lose her memory about Sofia's amulet or of their adventure.
- This is the first episode where the Secret Library has another visitor beside Sofia and Tilly.