While hanging out after school, Charlie and his friends discover the headquarters of the world's most powerful superhero hidden beneath his home. When villains attack, they must team up to defend the headquarters and save the world.

The Kincaid family - father Jack (Owen Wilson), his wife Lily (Jessie Mueller), and their son Charlie (Louie Chaplin Moss) - are camping overnight. Not far from them, U.S. Air Force Captain Sean Irons (Jesse Williams) drives by them, injured and in search of an artifact he has traced to that location. Jack finds him and tries to help him, and they discover a crashed aircraft in a nearby field. Emerging from the aircraft is a glowing orb that scans both men, choosing Jack as its guardian. The aircraft then self-destructs.

From here on, Jack becomes a superhero called The Guard. While he is regarded as an amazing protector of the world, his duties have caused him to neglect his duties as a father, so he and Lily get divorced, and Charlie stays with his mother.

Ten years later, Charlie (now played by Walker Scobell) is part of the baseball team with his best friend Barry Berger (Keith L. Williams). He has a crush on a girl named Maya (Momona Tamada), who used to go to school with him until she transferred out of the country, which Charlie believes is due to an incident he caused when they were kids, while Berger has a crush on another girl, Lizzie (Abby James Witherspoon). During the baseball game, Charlie goes up against a bully called Forrest Deal, AKA "The Sequoia". Forrest goes up to bat and swings the ball square into Charlie's stomach.

Berger's brother Big Mac (Kezii Curtis) drives the boys home. Charlie is set to go to Jack's house for his birthday, but he has also felt a distance from his father due to his constant excuses for missing out on his life. However, Charlie is also a big fan of The Guard. Meanwhile, CEO Ansel Argon (Michael Pena) of Argon Tactical has been wanting to use the source of The Guard's power for his own sinister schemes.

Lily drives Charlie to Jack's house, where he gives him a baseball mitt as a birthday gift...except it's for the wrong hand, as Charlie is a lefty. Jack makes a save with a Nintendo Switch, and the two spend the night playing together until Jack is called away for work. Charlie calls Jack out on his excuses and accepts that his dad will always disappoint him. Charlie pretends to call Lily to pick him up, but he is actually calling Berger so they can hang out.

Berger brings Lizzie and Maya over, to Charlie's surprise (and horror). While talking to Maya, Charlie accidentally opens a secret passageway that leads to Jack's basement. The kids head downstairs and discover what appears to be a lab filled with gadgets and a mission log of all of The Guard's heroic activities, leading Charlie to realize that his father is The Guard. The kids begin playing around with the gadgets. At the same time, it's shown that Irons has been working for Argon, trying and failing to locate The Guard's power source. Argon gives Irons until the end of the week to shut the project down and take his team with him.

The kids bring some gadgets to school to use to cheat on a test, and Charlie uses a wristband during his next baseball game to beat Forrest this time. However, their usage of gadgets, as well as Jack's G-Mobile, causes Irons and his team to pick up a signal and track the power source. The cops soon chase after the kids, but they manage to shake them off using more gadgets before they make it back to Jack's house. Jack leaves Charlie a message that he will be gone a little longer than expected.

The kids continue to hang out in Jack's lair. Here, Maya recalls the incident between herself and Charlie - when they were five and at a birthday party, they nearly kissed until Charlie chickened out and told the kids that Maya had bad breath, leading to her getting the nickname "Moldy Mouth Maya". Charlie thinks this is why she left school, but she clarifies that her mom had gotten a job transfer. Soon, Irons and his team show up at the house. They manage to make it into the secret headquarters and contact Argon to head over before Irons locks the place down. Charlie manages to grab the power source before the villains find it. While Charlie, Berger, and Maya hide, the bad guys find Lizzie, as well as Big Mac (who turns out to be Lizzie's boyfriend) since he was there to pick her up. The two are held hostage and order Charlie to give up the power source. Charlie puts his hand on the source and sees visions of war and destruction if he gives it to Argon.

Argon rigs the place with bombs before the rest of the kids come up with a plan. Berger uses a magnetic wand to hit Argon with a chair and throw him against the wall. They tie him up, and he explains how the power source works to them, as it is an alien artifact that adapts to the environment it inhabits. He wants to use the source to supposedly create weapons to solve the worlds problems, claiming that The Guard is selfish for keeping the power to himself. Charlie gives Argon the power source, but it turns out to be a fake that the kids made with a 3D printer. They cause a diversion by shutting off the power. Meanwhile, Jack sees security camera footage showing that Charlie and his friends are in danger, so he makes his way back home.

The kids fight the villains for the power source until Irons gets his hands on it, which also allows him to see similar visions to what Charlie saw, and he realizes Argon cannot be trusted, since he is fully prepared to kill the kids and leave no witnesses. Berger uses the wand to grab the power source and throw it into a portal connected to his locker at school. Jack shows up, and he begins to defend the kids. Argon gets hold of one of Jack's gadgets and forms his own suit, while Jack cannot suit up because he is not fully powered. Argon kidnaps Berger and forces him to show him where the source is at the school. Charlie leads everyone, including Irons, to work together to stop Argon.

The gang tries to 3D print jetpacks to head to the school. Lizzie tries to crack the system code while Charlie and Maya get trapped and nearly crushed by falling debris, at which point Charlie admits his feelings to Maya. Lizzie saves them, and they use jetpacks to fly together to head to the school, just as Argon's bombs go off and destroy the lair.

As a dance goes on at school, Argon brings Berger to his locker, but one of his teachers took it out of the locker and realizes Argon is a bad guy, so she helps Berger run. Argon forces the other kids in the gym to clear out as he tries to get Berger and the source. Charlie, Jack, and the others arrive for the showdown. Jack suits up and fights Argon, leading to the final confrontation in the gym. Just as Argon tries to grab the power source, Charlie throws a grenade at it, which blows up and sends Argon through a portal. After everyone turns out to be okay, Charlie dances with Maya, and they have their first kiss.

Charlie and Jack are later shown finally spending more time together. Charlie gets a distress signal, and Jack shows a new G-Mobile that Charlie can drive when he is older. They then head out on a heroic mission.

The ending text/clips show that Irons now works as the head of Mission Control for The Guard, and he continues his podcast on alien things. Lizzie and Big Mac are still dating, and are planning for their own first kiss. Berger uses the portal device to become a DJ. Maya is supposedly interning for the CIA (but it is not confirmed). The Kincaids are now a crime-fighting family. Also, Argon is alive but trapped in another dimension, still stuck in his power suit. (thanks to themoviespoiler)