Sofia and Miranda are riding in the Royal Coach to Dunwiddie Village. They arrive at the shoe shop that used to be their home. Sofia finds it just as they left it. Miranda shows her why she brought Sofia there: She made her daughter a pink dress and matching shoes to wear for her graduation. Sofia happily thanks her mother and changes into her new dress. Afterwards, the pair head back to Enchancia Castle. Sofia cannot believe today is her last day at Royal Prep. Miranda states that she cannot believe she went to Royal Prep or that she is a Queen, noting they have come a long way, to which Sofia sighs. Miranda asks Sofia what is wrong. Sofia explains that the other students want her to make a speech at graduation and she does not know the right thing to say is on such a big day. Miranda assures her she will do fine. Sofia's animal friends give her a tart. Cedric gives her a new tiara. Roland, Amber, and James are excited for graduation and the Royal Family heads to the coach. Chrysta and Skye show up and tell Sofia she needs to go to the Mystic Isles: Azurine has spotted Prisma on the Isle of Crystals. Sofia tells Amber, who agrees to cover for her. Sofia, Skye, and Chrysta then head to the Mystic Isles.
On the way, Sofia finds it is odd that Prisma would go to the Isle of Crystals since the Wicked Nine are all in the Protectors' Castle on the Isle of Protectors. Sofia quickly realizes that her sighting on the Isle of Crystal is a decoy to lure the Protectors away while she obtains the Wicked Nine. Chrysta assures Sofia that Garish was left behind to guard the Protectors' Vault which has an unbreakable lock. Sofia reminds her that Prisma has her Nessci-Key and might break into the Vault with it. Sofia, Skye, and Chrysta then head to the Protectors Castle where, sure enough, Prisma has broken into the Vault. Prisma then sets in motion her plan to absorb the magic of the Wicked Nine. Using their Enchantlets, Sofia and Chysta finally stop Prisma and recapture her. Sofia even gets her Nessci-Key back. Orion and Vega arrive with the other Protectors, who take Prisma away. Orion notes that the Locket of Vor has been destroyed and hopes Vor is destroyed too. Asking who Vor is, Orion explains to Sofia that she is an evil sorceress who tried to use her magic ring to conquer every single Realm eons ago. The Protectors stopped her and imprisoned her Spirit inside her Locket. They then divided up her Ring and sealed them inside the Wicked Nine. Sofia wonders if that was why Prisma was seeking the Wicked Nine. Vega reminds her that Prisma just wanted all of that power for herself. However, Vega also tells Sofia that if Prisma had completed the spell Vor would have been released. Sofia heads back to the Ever Realm to attend her graduation. At Royal Prep, everyone is wondering where Sofia is. Amber assures everyone that she is on her way, and arrives on Skye. Sofia finds Desmond, who is afraid to give his speech as the valedictorian. Sofia pulls him to the stage and they give their speeches together, to everyone's joy.
Back on the Mystic Isles, Twitch and Wormwood visit Prisma in her cell. Prisma is furious that she came so far just to fail. Suddenly, Vor's spirit appears before Prisma. It turns out that she did complete the spell and freed Vor. Vor hands Prisma her Ring. The second she puts it on, Vor enters Prisma and becomes flesh and blood. Vor reveals that she was using Prisma to get what she needed to resurrect herself. Now that she is restored she now plans to get her revenge on the Protectors and Sofia. Wormwood and Twitch pledge themselves to her. Vor turns Twitch into a unicorn and imprisons the Protectors.
Back at Royal Prep, everyone is preparing to go home. Vivian is sad since they will be going to different schools. Amber tells her they will all still be friends. Everyone then leaves to go to the party. Clover comes up to congratulate Sofia, when all the sudden she gets a call from Chrysta. She informs her of Vor's release and the imprisonment of the Protectors. Sofia asks Amber to cover for her, and then heads to the Mystic Isles with Clover on Skye.
On the Mystic Isles, It is revealed that Vor has Chrysta under a mind control spell and her call was to lure Sofia into a trap. Vor plans to conquer the Ever Realm starting with the Kingdom of Enchancia. Sofia, Clover, and Skye arrive at the Protectors' Castle. Orion and the other Protectors warn her about Vor's trap. Sofia and Clover stop in time but Skye falls into the trap: A magical barrier that keeps anything from getting out. Chrysta reveals the real purpose of her call earlier. Vega tells Sofia that Vor's goal is to spread evil everywhere she goes and is heading to Enchancia. Orion also warns her that Vor's going to seal the gate to the Mystic Isles and that if she does not get through in time she will be trapped in the Isles forever. With Skye trapped, Sofia turns herself into a Flying Unicorn and heads to the gate. Sure enough, Vor has cast a spell to seal the gates. Thanks to her Flying Unicorn form, Sofia gets through just in time. Sofia and Clover also find Vor just in time to see her conjure a legion of Crystal Soldiers to conquer Enchancia. Sofia and Clover feel they cannot handle this and fly off to go get help.
At Enchancia Castle, Baileywick and Cedric are getting the castle ready for the party. Miranda states that the decorations are almost as nice as having the whole family back together. She then states "Speaking of which: Where is Sofia?" Amber tells them she is running an errand. James notes that Sofia has been running a lot of errands lately. Meanwhile, Vivian, Desmond, Ruby, and Jade are on their way to the party at Enchancia Castle. Jade thanks Vivian for taking them to the party. Vivian tells her "Any friend of Sofia is a friend of mine." Suddenly, the quartet spots a pink unicorn. The Unicorn is revealed to be Sofia when she changes back into herself, to their shock. They ask her how she did that to which Sofia reveals the truth about her Amulet. She then tells them about Vor. The quartet agree to help Sofia as she is always been there for them. In Dunwiddie Village, the group unleashes an ambush on Vor and her minions. Vor overpowers them with her magic. Lucinda shows up to help but Vor is still too powerful. Sofia urges her friends to go to the castle and warn her family. However, her friends tell her to do that as they still want to help. Unfortunately, after Sofia leaves Vor easily subdues them. On her way back to the castle, Sofia tries to summon Minimus. Vor appears but Minimus rescues Sofia and flies her to safety. Vor attacks the castle. Cedric tries to fight back but Wormwood takes his wand, enabling Vor to take him and the Royal Family prisoners.
Meanwhile, Sofia and Minimus have retreated to the Secret Library. Sofia discovers Vor's conquest in the distance and feels like all hope is lost. Suddenly, her Amulet glows blue. The Library then follows suit. A book called The Tale of Princess Sofia is selected. Sofia rides the elevator down to receive the story. The book goes inside the pendulum and the Narrator tells Sofia her latest story: Once, there was a young girl named Sofia. She lived in the village with her mother, Miranda, and was happy, but missed her father, a sailor, who had been lost at sea. In the nearby castle lived King Roland II, who had wanted a family so badly he tried making a wish in a wishing well. The wish came true and twins were born. But the Queen fell gravely ill. The children were happy but never knew what it was like to have a mother. Until one day King Roland and Miranda fell in love and married. Sofia treasured her new family more than anything and she also became a Princess with the responsibility to protect her family, friends, and all those who dwelt in her Kingdom. So when the day came the evil Vor took everything Sofia held dear the young Princess was faced with her biggest challenge yet. Sofia sadly notes she already tried to give this story a happy ending and she failed. The Narrator reminds Sofia that she is the Storykeeper. The pendulum then turns into a man who is revealed to be the Narrator. He is revealed to be the first Storykeeper, who built the Secret Library centuries ago and filled it with many books that did not have happy ending. Sofia is one of the few who have been able to follow in his footsteps so magnificently. The Narrator, then reminds Sofia that all she loves is in danger and only she can save them. However, he also explains that she must be willing to face this evil on her own and be willing to make the ultimate sacrifice. Sofia sends Minimus to the harbor to wait for her and exits the Library to save her family. First, she turns herself into a Mermaid and swims back to the passageway to her room. She then enters her room with her shrinking ability. She then uses the Whichway Bow to find her family in Cedric's tower. Robin and Mia show up. After explaining what is going on, Sofia turns into a bird. After sending her bird friends to get help, Sofia flies into Cedric's tower. Sofia breaks the spell and, with help from her animal friends, breaks her family out. When they get to the harbor, Sofia tells her father she must stay. Roland insists on getting them to safety but Amber agrees with Sofia and, after Sofia turns into a Flying Horse, goes with her with the rest of the family following suit.
When they arrive back at the castle, Sofia and Amber, after remembering how Elena was trapped inside the Amulet of Avalor, decide to trap Vor inside it as well. Sofia activates the ability but Vor pulls Sofia inside with her, despite Amber's attempts to keep it from happening. With Vor trapped inside the Amulet, the effects of her magic get undone. Cedric arrives and subdues Twitch and Wormwood. Inside the Amulet, Sofia defeats Vor and frees Prisma, who apologizes for everything she has done. Sofia's friends, the Three good fairies, and the Protectors arrive at the castle. When Amber reveals what has happened, Cedric summons his parents and, with everyone's help, casts a spell that frees Sofia and Prisma, who surrenders to the Protectors. A relieved Miranda hugs her daughter. Orion declares Sofia Protector of the Ever Realm. Cedric gives Sofia back her Amulet. Roland thanks Cedric and tells him that from now on he will be called "Cedric the Great". Everyone then goes inside to celebrate.
- Ariel Winter as Princess Sofia
- Darcy Rose Byrnes as Princess Amber
- Nicolas Cantu as Prince James
- Travis Willingham as King Roland II
- Sara Ramirez as Queen Miranda
- Tim Gunn as Baileywick
- Jess Harnell as Cedric the Sorcerer
- Jim Cummings as Wormwood/Goodwyn the Great/Rex
- Jeff Bennett as Twitch
- Wayne Brady as Clover
- Ashley Eckstein as Mia
- Meghan Strange as Robin
- Russi Taylor as Flora
- Barbara Dirickson as Fauna
- Tress MacNeille as Merryweather/Winifred the Wise
- Megan Hilty as Prisma
- Paty Lombard as Vor
- Eric Stonestreet as Minimus
- Bonnie Hunt as Aunt Tilly
- Andrew Rannells as Skye
- Ellie Kemper as Crackle
- Ming-Na Wen as Vega
- Colin Salmon as Orion
- Jurnee Smollett-Bell as Chrysta
- Hugh Bonneville as the Storykeeper
- Diamond White as Ruby Hanshaw
- Isabella Acres as Jade
- Aden Schwartz as Prince Desmond
- Sabrina Carpenter as Princess Vivian
- Merit Leighton as Lucinda
- Jenna Lea Rosen as Princess Clio
- Coco Grayson as Princess Hildegard
- This is the sixth and final special in the series, followed by Once Upon a Princess, "The Floating Palace", "The Curse of Princess Ivy", Elena and the Secret of Avalor, and "The Mystic Isles".
- The Secret Library makes its seventh and final appearance in this special.
- Sofia's friends and family finally learn the entire truth of Sofia's Amulet.
- Sofia ultimately defeats Vor in the Amulet of Avalor.
- Sofia becomes the Protector of the Ever Realm.
- Tilly reveals that she knows about the Mystic Isles and the Protectors.
- During the narration of Sofia's tale, King Roland's first wish to the Wishing Well was revealed that he wanted a family, and his late queen makes a brief appearance. Sofia's birth father is also mentioned as well.