Mal, Evie, Jay, and Carlos may have once been the baddest of the bad, but their wicked ways are (mostly) behind them-and now graduation is almost here! But before the seniors can don their custom-designed caps and gowns, courtesy of Evie, they've got an epic plan to put into action. There are tons of villain kids on the Isle of the Lost who are eager for their chance to come to Auradon Prep-even Celia, Dr. Facilier's trickster daughter, wants in on the deal!-and Mal's crew is using their upcoming visit to the Isle to help make it happen. But Auradon's biggest threat is still at large?
During his early years in captive on the Isle of the Lost, former god of the underworld Hades commanded pirates and demons to dig underneath the island to find a way off of it, believing that the barrier could not go completely around the island; it would be a physical impossibility, even for magic. Humorously, his minions miss breaking through into the Catacombs of Doom, which was the only known path off of the island without opening the barrier. Fed up with the lack of results, Hades decides to turn the largest underground chamber into his new lair, developing a passion for rock music.
In the present, Uma has grown bored with living under sea, away from her army of pirates. She tried numerous times to get the attention of her top henchmen Harry and Gil through the barrier, but no success. Deciding to try breaking the barrier around the island, Uma commands her magic with all her might; the only thing she succeeds in is giving off a loud scream from her frustration. Her sea witch magic simply pales in comparison to the magic of Fairy Godmother. Hearing angry rock music coming from below the sea floor, she finds a crack and uses her magic to shrink down to fit through; the source is Hades. Uma reveals that magic is leaking in through the cracks, allowing a spark of blue light to return to Hades' long-burnt out ember; she coerces him into a partnership to get back at Mal and open the barrier.
In Auradon, the seniors go on an unofficial scavenger hunt called the Senior Quest. Items on the list consists of special items from the fairy tales of the kings and queens of the kingdom. To everyone's annoyance, Chad Charming wins; the last item on the list was to kiss a prince, but Mal, Evie and Lonnie each gave Ben a quick peck on the cheek, forgetting that he was already a king; Chad wins by kissing his own hand. The VKs are also doing well, with Carlos happily in love with Jane, while Mal, Evie and Jay have their futures to think about. Jay is scouted by several colleges to join due to his amazing skills in R.O.A.R. and Tourney; Evie is worried about where to live after leaving Auradon Prep; Mal has to worry about visiting all the kingdoms and learning about their different cultures in preparation for her future as Lady of the Court.
When Evie voices her concern that only Dizzy has applied for relocation to Auradon, Mal decides to visit the Isle first on her royal tour of the kingdoms to try getting more kids interested. Mal brings it up during the royal meeting, surprising Beast, Belle and Fairy Godmother with the idea of going back to the Isle. Ben reminds them that the Isle is Mal's home and that the program has been at a standstill since the first kids from the isle came over; Freddie and CJ were unofficial additions, thanks to Jordan accidentally granting Mal's wishes. Mal reassures them that she will be safe; Evie, Jay and Carlos will come with her, ensuring safety in numbers. And just like last time, the amount of kids being brought over is four; there can't be too many, or it risks letting them run wild in Auradon (which is dangerous and would make the first four VKs look bad by association with the Isle).
Arriving at the Isle of the Lost, Mal, Jay, Evie and Carlos visit Bargain Castle, which has been left alone since their last visit to the island, two years previously. Deciding to speak to the island as her mother once did, Mal is initially mistaken for her (much to her own annoyance as she obviously doesn't have horns) due to being in the shadows of the awning. The VKs ask for the children of the island to listen to them, explaining that life in Auradon is much better than on the Isle and that they are offering the chance to join the rest of the world that they were once given. However, none in the crowd are swayed by their words, much to their disappointment.
The following day, they are forced to bribe Dr. Facilier into cooperating with the VK program by offering to put in a good word for Freddie's sister Celia to fill one of the four slots available. They immediately spot Celia, who had been listening in on the conversation in the headmaster's office; the other VKs leave to spread the word, while Evie gets her fortune told by Celia. It tells of Evie's past and present, which lead to her good life; however, the prediction of the future forewarns that while change may come, it could be disastrous. Skipping class, Celia wanders through the underground tunnels, hearing voices come from Hades' lair. She recognizes the other voice as Uma's as she listens in, learning of their plan to go after Mal; it's the disaster she foretold.
Mal, Jay, Carlos and Evie visit former teachers Lady Tremaine, Gaston, Madame Mim and Mother Gothel respectively; they ask them to inform students about the VK program playing on their weaknesses to get cooperation. Unfortunately, the only ones who show up for the presentation are the irredeemable Anthony Tremaine, Mad Maddy and Ginny Gothel; the cowardly LeFou Deux was forced to come. To the VK's frustration, the kids all fell asleep during the slideshow about Auradon Prep. After learning that magic is discouraged and regulated, the villain kids ask about embezzlement, poison and revenge; however, these are not allowed in Auradon. They leave, severely dissatisfied. At wits end, Carlos suggests they seek out the Anti-Heroes Club to fill the slots; however, he is informed by Jay that Gaston told him that Dr. Facilier found out about the true intentions of the club and disbanded it, leaving them fearful of continuing to be good. They also cannot seek out professor Yen Sid for help in encouraging the club to apply, as he's on sabbatical in Auradon.
The VKs have dinner at their hideout, grateful for Evie packing food and other luxuries from Auradon. They are visited by Celia, who is mesmerized by the delicious food. Sharing with her, the VKs are warned by Celia that the fortune that she predicted - disaster - is coming to fruition. She tells them that she heard Uma's voice in Hades' lair, but is met with disbelief as no-one can get through the barrier after it was strengthened after Uma's escape the last time. They give Celia the leftovers and send her on her way; Celia is not that concerned, as she did deliver her warning to them. What comes of it is all up to the VKs.
That night, Mal has a dream of her childhood, being commanded by a voice to go somewhere. In a trance, Mal sleepwalks out of the hideout and sees Dizzy, Gil and Harry, who tell her to keep walking. This leads Mal to a forest where a blue light calls out to her; Mal recognizes it as the voice of her father. However, Mal quickly gains some clarity and exits the forest as fast as she can. She finds a confused Dizzy, who suddenly speaks in Uma's voice while being joined by Harry and Gil, who Uma uses as speakers to declare her revenge. Mal jumps off the pier to escape them. Unfortunately, this was what Uma wanted; the three had just been an illusion. Mal becomes trapped at the bottom of the ocean, where the magic flows in. Allowing Mal to breath in water, Uma plays mind games with Mal, trying to steal the remote to the barrier from her.
Having discovered Mal's absence, Evie, Carlos and Jay had arrived in time to see Mal jump in the water, Jay attempts to save her, only to be repelled by a wall of fish. Realizing that Uma has found a way to bring some magic into the barrier, Evie realizes that there's a way to find Mal. Evie leads Jay and Carlos to her old home, as the Evil Queen is out playing Apples in Apples with other hags. Disguising herself as her mother, Evie scares the vultures into flying away from the spare key. Inside the castle, Evie finds the Magic Mirror and thanks to her disguise, is able to command the weak magic filling the broken mirror to reveal Mal's location. It shows a still-dazed Mal about to open a door on a sunken ship. The moment Mal opens the door, Uma pops out to steal the remote; however, Evie uses the magic mirror as a portal and pushes her away, sending Mal back to the surface. Everyone suddenly forgets most of what happened. Mal wanders back into town, while the trio escape the Castle Across the Way just as the Evil Queen returns home.
Elsewhere, Celia tracks the cracks of magic in the underground tunnel. Realizing that the presence of magic, though minimal, will allow her to use it, Celia uses her tarot cards to predict her future; there's a great chance on the horizon for her. However, Celia realizes that there's a great threat to having even a small amount of magic leak into the barrier, as the villains can slowly siphon it off until they have enough to break the barrier. Celia calls upon her own shadow, her own friend on the other side, commanding it to cover all the cracks letting magic into the barrier. Celia is confidant that she will earn a place in the VK program for this, though she decides to keep it a secret until she can use the action to gain favor.
In their hideout, the VKs are feeling down as they have failed to recruit anymore kids for the program. However, the four of them realize that they've been in Auradon too long; they have been trying to use things that appeal to good people to get the kids' attention. Instead, they should be trying to appeal to their selfish desires. Carlos remembers that VK Day is about to be announced, which is a holiday to celebrate children of the isle like them. Everyone agrees that the best thing to do is make a big enough deal about getting to go to Auradon to stir up enough interest in the program.
The next day, they tell Dr. Facilier about a "new development" in the VK program. They explain that the next kids to be selected will be honored with a celebration spectacular beyond anything that came before, including a parade, a band and a kingdom-wide feast - certainly rolling out the red carpet. Dr. Facilier agrees to actually spread the word this time. They tell him that the selection will occur in a few weeks; Mal, Jay, Evie and Carlos return to Auradon. The following day, Dr. Facilier announces to the school that the next four chosen to leave will be immortalized as the Isle's finest. The children are surprised; they believed that Mal, Evie, Jay and Carlos had been pulling a prank, but now realize the chance at a life outside of the barrier is closer than they thought. Dizzy and Celia grab applications; Harriet Hook grabs one for herself and two for Squeaky & Squirmy Smee; even Anthony Tremaine, Mad Maddy and Ginny Gothel grab applications.
In Auradon weeks later, Evie provides custom caps and gowns for the seniors. Doug takes inventory on her income, which Evie notices is enough to buy a home. At a dinner party to celebrate the end of the school year, Mal informs Ben of the Isle adventure. Evie also reveals that she bought a cottage to live in with the help of Doug and his uncle; it's also spacious enough to host the next quartet of VKs for the summer. Carlos is excited at the idea of being senior VK to the new ones. Jay also gets into Sherwood Forest University, with his acceptance letter even being signed by Robin Hood himself. Best yet was the traditional senior prank, which the VKs mastermind: a crashed pumpkin carriage, vending machines filled with water bottles with goldfish in them, plastic wrap and balloons in the halls, chairs replaced with thrones upside down on the tables, a "For Sale" sign on the roof and a New Orleans jazz band following Fairy Godmother around all day.
Come graduation, Carlos and Jane reveal Ben tasked them with setting a streaming signal to the isle, allowing cameras and phones to work. The graduation is being shown live to encourage applicants. Mal receives the award for Most Improved Student; Jay for Best Athlete; Evie for Diligence. Once rotten to the core, the trio now have bright futures ahead of them. Ben reveals a large amount of applications came in.
Elsewhere, Uma had been knocked out as well outside the barrier; her memory is also hazy, except for the fact that she failed to beat Mal yet again. Her attempts to get back in are blocked by Celia's shadow, unknown to her. Convinced Hades has double-crossed her and sided with Mal, Uma swears he will pay along with her nemesis.
- Part of the novel incorporates Under The Sea: A Descendants Short Story.
- New characters are introduced:
- The plot hole in Descendants 2 - what happened to Maleficent - is finally addressed. Mal has no clue where her mother is.
- A tradition at Auradon Prep is the Senior Quest, which is a sneaky scavenger hunt to collect items from around Auradon within a few hours.
- Quest Items:
- A shard of Cinderella's broken slipper.
- A flower from Aurora's rose garden.
- A lock of Rapunzel's hair.
- A banana from Terk's fridge.
- One of Rajah's collars.
- An apple from Snow White's orchard.
- Slice of cake from Mad Hatter's tea party.
- One of Ariel's thingamabob's (corkscrew).
- Make a beignet using Tiana's recipe.
- Kiss a prince.
- Hum "Be Our Guest" at the finsh line.
- Quest Items:
- The "good" equivalents of Anthony, Ginny and Maddy's interests would be finances, medicine and law enforcement.
- Razoul has retired from security in Agrabah and is heading the Agrabanian R.O.A.R. team.
- Little John of the Merry Men is coach for Sherwood Forest.
- The sister of Fairy Godmother is a coach for the Magical Institute of Teaching - M.I.T.
- Yen Sid is in Auradon on sabbatical at the time the VKs visit the island.
- Yao is captain of the pro R.O.A.R. team for Northern Wei.
- Evie shows a complete acceptance of commoner life; rather than buy a castle, Evie gets a cottage.
- Doc is a realtor.
- Doug has a brother named Derek.
- Doug's other cousins include Shy, Crabby, Cheerful, Snoozy, Doc II, and Gesundheit "Gus".
- The VKs have a text chat room; their online names are C-Dog (Carlos), It's Jay (Jay), Malevolent (Mal) and Princess Evie (Evie). Jane has an online name as well; it's Jaaaaane.
- It's revealed that Gaston, Madam Mim and the Queen of Hearts are teachers at Dragon Hall.