Lincoln Loud is excited to celebrate his Gran-Gran
Myrtle's marriage with his grandfather,
Albert. Knowing of, and fascinated by, Myrtle's past life as Agent 28, a
secret agent who defeated a supervillain named Dr. Rufus Dufus in her youth, Lincoln insists on bringing his
David Steele spy equipment, despite Myrtle wanting to focus on the wedding. She entrusts Lincoln to protect her wedding rings, though he treats it like a spy mission. The Louds head to a tropical island to celebrate the wedding; on the flight, Lincoln meets Fifi, a woman annoyed by his family's antics. The family reaches the Thunderball Resort, run by Flip's cousin Flop Phillipini. Later that day, Lincoln witnesses suspicious activity while on a fishing trip with Myrtle and Albert. He attempts to take photos, but is ambushed by henchmen. Myrtle is able to fight them off, but she loses Lincoln's camera in the process. Returning to shore, Myrtle tells Lincoln to not be involved in any more missions, and that they would be leaving the island, out of concern for her step-grandson's safety. However, Lincoln insists on going anyway, telling Myrtle's superior, X, that they would undertake the mission behind his step-grandmother's back. Shortly afterward, he runs into Fifi again, who gives him his camera. As night falls, Lincoln and his friend Clyde trace more suspicious activity to a restaurant called Rubiner's. Seeing Lincoln sneak away, Myrtle ropes a bellhop named Owen into masquerading as her and partaking in family activities while she follows her step-grandson to Rubiner's. While there, Lincoln manages to overhear some henchmen planning to send rocket parts to Dufus, but is interrupted by Fifi and eventually Myrtle, to whom he confesses that he accepted the mission on her behalf. The two are spotted; they manage to defeat more henchmen together and escape from the restaurant, but Dufus' old henchman Ham Hand captures Myrtle while Lincoln escapes. On the run from Dufus' henchmen, Lincoln is saved by his family, who find out the local police station is also run by Flop. A determined Lincoln rallies his family into helping him save Myrtle; the next day, Lisa, having prepared in advance, gives her family spy gadgets. Inspired by a
David Steele comic, Lincoln deduces that Dufus' base is in a nearby mountain. Fifi, appearing in a rental buggy, accidentally gives away their location, and the Louds are forced to lose their pursuers by the beachfront. Reaching the secret base, the Louds sneak in and split up to cover more ground. Myrtle is able to escape her cell and reach the rocket platform while her family finds her cell, which they are locked in. Myrtle fights Ham Hand while the jail cell containing the other Louds is raised up and connected to the rocket, distracting her long enough for her to be held captive by Ham Hand. Fifi reveals herself to be the mastermind - and Dufus' wife - having used Lincoln as bait to lure the Louds into her trap with the intention of launching them all into space. With the rocket about to launch, Myrtle is able to defeat Ham Hand, but Fifi, having gained possession of the wedding rings Lincoln was meant to protect, drops them into the ocean. An enraged Myrtle is unable to defeat Fifi as the rocket launches into the sky. A heartbroken Lincoln takes responsibility for leading them into Fifi's trap. Myrtle points out that they can all use the rocket's septic tank as an escape pod, but the tank would need to be closed from outside. Lincoln takes initiative, staying behind - much to the others' horror - while the others are launched back to the ground. The other Louds are heartbroken about Lincoln's sacrifice. After reading one of Lincoln's
David Steele comics, Myrtle has the family return to the rocket console to establish a connection with Lisa's nuclear fusion laser, which strikes a satellite and hits Lincoln's rocket, causing it to blast back to Earth and into the nearby sea. Albert swims down to the rocket, which is already breached with water, and frees Lincoln. Albert brings Lincoln back to shore, to the others' relief, and the family shares a group hug. Myrtle and Albert reconcile, announcing that the wedding will go on. Lincoln brings Myrtle replacement bamboo wedding rings, having sold his spy watch. With Fifi and Ham Hand arrested, Flop officiates the wedding, and later that night, the Louds party on. ==Voice cast==