During the vacation on four days and three night, darlyn and friends trip to a small town called baler to fight greatest sin.
Prologue begin with mia is caring an animal for festival summer, olivia cal for help, when the children are kidnapped. Mia must rescue children.
The movie begin with mia and her friend are headed to museam, but the children is safety by a museum chaperone named Sarah jackson takes mia, her friend and a group of children on a tour of Heartlake folk myths and legends. She tells them the story of the Heartlake town of Baler aroua from the Small town crops, which holds every story in the world.
The mermiads, queen belle and king Arthur ruler of the Kingdom of the Joyfull ocean, and eris the witch Queen, ruler of the kingdom of the Underground, set a wager at Baler aroua's Day of the Survival festival after seeing six heroes, Wu, garmadon, Emma, Misako, mai and tom, competing over a young solider and Prince named Heather dawn and Prince Allen. king arthur and Queen belle bets that wu and garmadon will join Heather and allen, whil eris bets on Emma and Misako. If Belle wins, eris can no longer interfere in mortal affairs, but if eris wins, she and Belle switch realms. However, eris cheats by force emma and Misako giving Dareth emma stronger's Amulet of Endless armor, which grants the wearer invincibility. Heather frees a herd of pirahnas from being slaughtered, angering Heather's mother in laws, who sends her son and daughter in laws away to a boarding new homelands in Ninjago. While saying their goodbyes, Allen gives emma and Misako a baby pirahna and fish from earlier names after dianne and window and Heather gives wu and garmadon a gift for a new student name after darlyn and lloyd which is engraved with "Always remind themsevles".
Years pass and kai's adoptive brother and sister, lloyd and darlyn's musician dreams are suppressed by wu and garmadon, who trains them to become a mousehunters like the rest of mousehunters, while Angela's adoptive sisters dianne and window becomes the revered town two hero with the amulet's aid. On the day of Heather, allen and their children Chris and caroline return after darlyn picture her self and her family in several places, a celebration is held culminating in darlyn's first mousefight darlyn defeats the mouse but refuses to kill it, disappointing Wu, garmadon and the crowd but impressing Angela and her friend. That night, Kai sing a song for Angela with help of the former burglar, alex the his mates who got hit by flower vase. Angela was pressured by her father to close friend of zane, Kai is jealous of his rival, Zane and they fight each other, but it stop by Angela, the heroes walk to hotel room.
That day, darlyn and her family go to baler ditumabo falls, but the water is too cold as lloyd, Cole and zane laugh at darlyn, Kai and jay in cold water, meanwhile the bandit army appears as scaring the people away, manolo and friends are her to save them, but the waves appears as swallow bandits, the heroes return to hotel for rest. The next day, the heroes goes surfing, but get busted by wu for not do fun. After surf, darlyn and her family go to next hotel. As darlyn stay to rest.
That night, Angela is pressured again by king allen and Queen Heather dawn to be close friend, love interest and partner, so that Dareth will stay and protect Baler aroua from the queen of the chrysalis, scorpion and the king of the clockwork, bear king, though she and her friend is conflicted by her feelings for Kai and friends. The lover teammates crew and the teammates crew meet that night to profess their love for each other, but they are interrupted when a two Christmas demon character, sent by eris, bites Angela once and seemingly kills her. Despondent and blamed for her death, Darlyn and friends is conned by eris into being sent to the afterlife: the Christmas bites them twice, killing them all.
The teammates crew arrives in the Land of the Kingdom of the Joyfull ocean where she reunites with kai's close friend, Olivia katherine and the illustrious deceased and non deceased heroes and redeemed villains members. They travel to King Arthur and Queen belle's castle to seek Angela, but only find eris, who explains the bet to an outraged teammates crew and that the two Christmas character put Angela in a coma, but killed them. When Angela awakens, she learns of The teammates crew's death and solemnly accepts Dareth's proposal. The teammates crew, agent of the rescue, Guardian of galaxy, Olivia, Nima, her family and David jones, Emily's befriend, travel to the Cave of Underwater to reach king arthur and his family. They meet the amiable Cryptkeeper, who oversees the lives of everybody in the living world. After seeing that Darlyn's story in the Small town crops is blank (due to eris's meddling) and can be rewritten by darlyn's own actions, the Cryptkeeper takes them to the Kingdom of the Underground. The teammates crew exposes the cheating to Arthur, who furiously summons eris. Another deal is negotiated; The teammates crew's life will be returned if she completes a challenge eris sets them, but if she fails, eris will rule both kingdoms and the teammates crew will be forgotten. Eris, believing Kai's fear is mice and rats, manifests every rodents the Heroes and good villains ever fought which combine to become one giant one for The teammates crew to defeat.
In the living world, bear king and scorpion, who previously owned the Amulet, leads their army to Baler aroua to find it. Bear and scorpion kills Angelo and his friends, who joins the sins fighter to watch The teammates crew fight. Darlyn, realizing her fear is actually being herself and harm the rodents, again refuses to deliver the finishing blow, instead singing an apology to the grudge-filled spirit asking it to forgive his friends's transgressions, which it does by peacefully dissolving away. Impressed, the deities grant The teammates crew their lives back and send them and sins fighters's spirits to get their lives back to the living world to defeat bear king and scorpion. Darlyn, lloyd and kai are almost killed again when scorpion blows them all up but are saved by the Amulet, which Dianne gives her sister at the last second. Window returns it to eris and resolves to be a hero of dianne's own accord, while Chris, caroline and their love interest, Cassie and jack's wed happily as Arthur, bell, jess and eris reconcile.
The story ends, and as mia, her friends and the children leave the museum for return home, Sarah jackson, old woman, sarah's father and Sarah's mother from earlier reveal themselves as Princess jess, eris the witch queen, king Arthur and queen belle.
Epilogue, the post credit scene darlyn return to her home with her family, while rodents arrives, causing terror kai screams.