In 1994, a man named Lynn Loud meets a crossing guard named Rita while walking down the street. They experience love at first sight, and Rita invites him out on a date. After many dates, the two of them get married, then buy a suburban house for them to live in. During their first year in the house, they give birth to their first child, a daughter named Lori, followed by four more daughters named Leni, Luna, Luan, and Lynn Jr. After they have five daughters, they give birth to their first son, an albino boy named Lincoln. After Lincoln, the two of them go on to have five more daughters named Lucy, Lana, Lola, Lisa, and Lily. In the present day (2017), Lincoln, who has become the family's "man with the plan" as he likes to call himself, teaches Lily how to create and execute a plan as they decide to be the first ones to eat their father's breakfast burritos. While their sisters end up getting ready before them, Lincoln is able to prevent them from further intervening in their plan by sending them out of the house against their will, noting how the key to having a plan is to always have a backup plan. After they begin eating their burritos, they soon learn that all of the girls have activities that day. Lincoln helps out his sisters, and they take home several awards after they finish their activities. That night, when the family goes out to eat, the girls pose for a group photo where they show off their awards. Lincoln attempts to join them in the photo because he is also part of the family and did plenty of work to help them, but he gets abusively shoved out of the frame and locked outside, and the public completely ignores him in favor of his sisters. This causes Lincoln to wish that he was special, and he decides to visit his best friend, Clyde. During the visit, Clyde talks about how he had recently taken a vacation to France, where he learned that he was descended from a long line of bakers and found out that he was just as skilled as his ancestors were. This inspires Lincoln to ask his family to take a vacation to their place of origin. After collecting a DNA sample from Lynn Sr., Lisa concludes that their family originated from Scotland, so the family takes a vacation there. Upon arriving in Scotland, they find themselves in a small town called Loch Loud, which, while still decently inhabited, is relatively dysfunctional and noticeably requires improvement. The Loud family soon learns that they are descended from royalty from a man named Angus, who works as the groundskeeper for their ancestral castle. After taking them to the castle, he introduces them to the castle's caretaker, Morag, who noticeably enjoys being in a peaceful and quiet setting more than anything. He then invites them to stay in the castle while on vacation, given that their ancestors lived in it. They accept the offer, and once they begin exploring the castle, Angus decides to talk to Lincoln after noticing his fascination with fame and recognition. He explains that the royal family was very special and well-known among the townspeople, but the most special of all of the Loud family's ancestors was Lincoln's double, who was the duke of the town. After being taken to the duke's room, Lincoln decides that he wants to be just like his ancestor. Suddenly, Lynn and Lana arrive in the duke's room, having noticed the cave near the castle with a prominent dragon motif, and ask Angus to explain why it exists. While Angus takes the two girls to the cave, he explains that dragons are an important part of the town's history, and the royal family owned a real dragon back in their time. Soon, he discovers that the only dragon egg left in the cave has hatched, meaning that a dragon is on the loose. After they catch up with the dragon, they bring it to the castle, where Lily names it Lela. Suddenly, Lincoln emerges from the duke's room, wearing the duke's clothes and having decided that he wants to become the duke. At dinner that night, Angus explains that the only way Lincoln can become the duke is by making an effort to help out the townspeople. The next day, Lincoln helps out the townspeople (with help from Angus, as he needed to learn along the way that being the duke is about more than being popular and self-entitled), which manages to convince them that he would make the perfect duke. However, Morag, who wants to regain her peace and quiet after the Loud family leaves, blurts out that Lincoln is a tourist, which causes the town's citizens to abandon him. Lincoln then convinces his family to move into the town, which they agree to after being manipulated by him, and he is later crowned as the duke. Soon, as Lincoln is enjoying himself, living as his ancestor did four hundred years before him, Morag sneaks away from the castle to the highlands, where she completely snaps and openly expresses her hatred for the Loud family. She then digs up her distant ancestor, Aggie's, journal, where she uncovers a dragon-controlling gem. She plots to use it to exile Lincoln from Scotland, just as her ancestor did to her duke. That night, Lucy summons the ghost of her distant ancestral double, Lucille, who explains the true story of the royal family's departure from Loch Loud: They left for a temporary amount of time, but upon returning, the duke's dragon friend, Lolo, attacked their ship. Lucille had assumed that Lolo had deemed her brother, the duke, to be worthy no more, and he thus handed over his crown before sending them all away to a new land. What none of them knew was that Aggie, the castle's historical caretaker, was controlling the dragon with a gem, and she had plotted to exile the duke from Scotland out of frustration for never getting a moment of peace and quiet as a result of the large royal family's presence. After Lucille finishes explaining the story of the royal family's departure, Morag arrives back at the castle, then eavesdrops on Lincoln, where she learns that his greatest weakness is being stuck in his sisters' shadows. She then decides to motivate him to abdicate the throne, then usurp the throne and become the duchess. The next day, she invites Lincoln's sisters to perform in front of the town, since their ancestors were widely known for their many areas of expertise. While Lincoln is cleaning out chimneys throughout the town later that day, he notices a flyer promoting his sisters performing in the town, which causes him to lose his confidence about being special and popular due to his royal status. Morag then convinces him to ride the dragon like his ancestor once did, and Lincoln agrees to the offer. Sure enough, the townspeople admire Lincoln as he rides the dragon, but during his dragon ride, Morag takes control of the dragon using the gem she found in Aggie's journal. With Morag now in control of the dragon, property destruction ensues throughout the town, and the townspeople blame Lincoln for it. They later form a mob against him, which motivates him to abdicate the throne and leave Scotland. After the Loud family leaves Scotland, Morag usurps the throne and becomes the duchess. Angus and the townspeople begin opposing her because this action is unlawful (as the Loud family did not give her consent to become the duchess after they left, as she claimed soon before her coronation), and not wanting to be opposed, Morag attacks the town using the dragon. Meanwhile, Lucille learns from Aggie's journal that Aggie betrayed the royal family back in their time and was responsible for the original duke abdicating the throne, and she soon reveals the truth to the current Loud family. After telling them that Morag is attacking the town and sending all of its residents into exile, they return to the town and battle against her. Lincoln soon confronts Morag face-to-face and begins to battle for the crown. However, they lose focus of where the dragon (which they are standing on) is going, and they crash through the castle walls. Morag prepares to kill the entire Loud family and Angus, but Lincoln is able to prevent this by attracting Morag away from them by taking the crown with him. During Morag's last-ditch effort to murder Lincoln, Lincoln uses a magic trick to replace the dragon-controlling gem with a burrito, then destroys the gem so Morag cannot control the dragon anymore. After another fight over the crown, they fall off a cliff, but the dragon manages to prevent them from dying. The dragon takes Lincoln back to the castle, but leaves Morag stranded on an island populated by noisy seals. After Lincoln arrives back at the castle, he decides that he does not deserve to be the duke and instead decides to crown Angus as the duke, since he knows the town better than anybody else. After doing so, the Loud family helps to rebuild the town together with Angus and the ghosts of their ancestors, then they leave Scotland to return back home, satisfied with how they have helped the town.