Around 5 years ago...
In outer space, two ships are battling. Slugging it out with laser blasts, the larger ship; 'the Chimeran Hammer', seems to be on track to become the victor - with its commander Vilgaa, declaring that she has come too far to be denied, with no one being able to stop her from achieving her goal.
Meanwhile, on Earth, in a city that would come to be called Bellwood, a 10-year-old girl named Jen Tennydottir is sitting in school, mischievously folding and throwing a paper airplane, interrupting her teacher's lesson. Waiting, she urges time to pass faster while her teacher makes mention to the class of summer school - an idea Jen finds humor in. The bell finally rings, allowing the class to go free, but as she's leaving the teacher scolds Jen about her paper airplane-antic. Outside, school bullies Cassia Maria and Jessy T. are trying to steal money from a girl named Gemma. Jen stands up for Gemma, and ends up knocking them both out. Jen's grandmother, Maxi Tennydottir, arrives in her RV, the Rust Bucket, to take Jen on their anticipated summer road trip. On board, Jen's excitement drops as she spots her cousin, Glen Tennydottir, who claims that "someone" persuaded his father this would be a good experience for him. It becomes clear, the only mutual interest they have is not wanting to spend time together. They are seen bickering while the RV drives off into the distance. When they arrive at the campsite of their first destination, Jen and Glen learn that Maxi has a plan to take them on a flavor ride which includes eating 'live meal worms' - the two learning another common interest, the need to avoid their grandma's cooking at all costs.
Back in space, the space battle continues, where Vilgaa orders her Drones to board her opponents' ship, which is able to project a blast at the Chimeran Hammer, engulfing Vilgas in a fiery explosion. In retaliation, the Hammer destroys the smaller ship with a blast, but not before it's able to jettison a pod that falls to Earth.
Getting away from her cousin, Jen goes for a walk alone, until she sees a what she thinks is a shooting star. The "star" is revealed to be the pod jettisoned at the start. It twists its trajectory and hones in on Jen and she barely manages to escape its destructive path. Edging closer to the crash site to get in a look (at what seems to her to be a satellite), she falls into the crater created from the impact. The pod opens itself to reveal to Jen a black watch-like object with purple accents: the Omnitrix. Surprising her, it latches itself onto Jen's (left) wrist. After flailing with it, and multiple failed attempts to get it off, the faceplate emblem opens up, revealing a silver dial surrounding a purple hourglass, and projects a hologram of what we later know is of ten aliens. Thinking this is cool, Jen slowly pushes the faceplate back down and as time slows around her, has her body transformed into the water and sandstone body of Beach-Party - all blanketed in a flash of purple light. In this alien form, Jen screams and runs around in a panic, terrified to see herself in this new body, but suddenly calms as she realizes that she isn't in pain or anything. She decides to test her power out, launching a stream of water at the sky, ending in her accidentally creating a chaotic storm.
Glen and Maxi notice the strangely area-specific storm from the campsite. Thinking Jen may be in danger, they both grab an umbrella and rush to the scene in attempt to save Jen, only for Glen to stumble on Beach-Party flailing her arms attempting to stop the storm, which is only making it worse. Beach-Party tries to reason with him, only getting herself hit with the back end of the umbrella; with Glen lifting his axe and warning her to stay down. Blasting his pants with water, and getting a cheap laugh in the process after calling him a name, Glen's convinced this is really Jen. He asks what happened, and Beach-Party spews the whole story in a matter of seconds, finishing just as Maxi appears. She is just about to retell the story again, but Glen interrupts and reminds them of the situation at hand. Maxi has an idea to get her granddaughter to try and absorb the moisture from the clouds and see if it works;it does, but a large portion of the forest gets destroyed just as the park rangers show up.
Back on the Hammer, Vilgaa's body is fairly torn up, with both her arms and left leg missing, relying on life support in a large tank. Learning of the Omnitrix's situation, she sends a drone of hers to retrieve it. Meanwhile, Beach-Party recounts her story to Maxi. Glen asks Maxi if she thinks that Jen will stay this "freakshow" forever, with Maxi instead grimly correcting him to 'alien'. Interrupting them, the Omnitrix times out and Beach-Party reverts back into Jen, hidden in a flash of yellow light. Maxi leaves, telling her not to fool around with the Omnitrix while she goes to investigate the spot of the crash for answers. As she leaves, we see Vilgaa's drone destroy the empty pod, and separate itself, sending two smaller drones off of its shoulders to search and destroy.
Disobeying her Grandma's orders (who's now come across the drones' destruction), Jen decides to try the Omnitrix again, and Glen watches on as she transforms into reptilian Gator-Ray. In this alien form, Gator-Ray quickly discovers her heightened senses and permanent X-ray sight and disappears into the woods to have some fun - contrary to Glen's warning. While perched on a tree limb, she senses two drones nearby. She leaps out of the way moments before a laser obliterates the branch she was previously on. One of the drones sets out after Gator-Ray, who manages to defeat the drone by crashing it, but then -with the Omnitrix timing out again- she reverts back into the un-powered Jen, and gets knocked to the ground and targeted with the laser from the second drone. Glen appears and hits the second drone with a shovel repeatedly, destroying it and saving Jen.
With the trio returning to the RV, Maxi lectures Jen for misusing the Omnitrix but makes the decision to help her granddaughter learn to use it. At that moment, a call comes through on the radio searching for help from the robot drone, now attacking several parked RVs nearby. Realizing the drone is after her and the watch, and that she could use these new powers to become save them, Jen makes the decision to become a hero.
As they set off to arrive at the scene, and with a nod of approval from Maxi, Jen transforms into Gumstick. in this alien form, Gumstick tells Maxi and Glen to get the campers to safety, while she fights the robot. The drone scans her, and - seeing the emblem of the Omnitrix on Gumstick's head - attacks. Gumstick survives the robot's blast unharmed and attacks back, but it dodges and pins her to the ground. The same ranger from the forest is escorted by Maxi and Glen as she sees the battle, which is now nearing closer to them. Gumstick gets thrown by the Drone, almost hitting the ranger, and - seeing a tree collapsing towards Glen - stretches
her arm grabs it and moves it. Interrupting Jen's heroic feat (which now puts her 'even' with Glen), the robot grabs her and removes her arm; however Jen's able to regrow it and return the favour and destroy its hand. Goading the robot to fires a laser at her chest, Gumstick absorbs it, redirects it into her fist, which inflates, and punches the droid, destroying the robot with its own blast.
Glen and Maxi cheer for Je... 'Gumstick', as she makes a quick escape. In space, Vilgaa is told that her robots have failed her - swears to destroy whoever kept the Omnitrix away from her.
Jen decides to use her new powers to cause some self-justice while Maxi and Glen are asleep, and transforms into Standstill. In this alien form, Standstill rapidly slows time and runs to Bellwood and back, getting her payback for the same spectacle Cassia and Jessy T. tried against her and Gemma. When she arrives back at the campsite the next morning, Maxi and Glen are packing up to continue the trio's road trip, helping pack up before reverting back into Jen, she says that she needed to take care of a few things at home. Meanwhile, the two bullies are hanging by their underwear from a tree, unsure of how they came to be there.