Otis Haywood Sr. runs a ranch in Agua Dulce, California for training and handling horses used for film productions. One day, he and his son Otis Jr., or "OJ" are outside when small metal items start falling from the sky. A nickel penetrates Otis Sr. through his eye, killing him.
Six months later, OJ and his sister Emerald, or "Em" are struggling to make ends meet. They get fired from a set after their horse, Lucky, reacts violently to its own reflection in a chrome ball utilized for visual effects. To make some money, OJ sells some of the horses, including Lucky, to Ricky "Jupe" Park, who runs a small Western theme park called Jupiter's Claim next door to the Haywood Ranch. Ricky is a Former Child Star who was on a sitcom in the 90s called Gordy's Home! about a family with a pet chimpanzee named Gordy. During the filming of a Birthday Episode, "Gordy" reacted violently to the sounds of balloons popping and attacked his human co-stars.
Later that night, Emerald talks about Jean Jacket, a horse that was supposed to be hers before Otis Sr. sold it. The power at the Haywood house goes out briefly, and OJ investigates one of the horses, Ghost, reacting to an unknown presence. He briefly spots a UFO flying through the sky behind some clouds. They go to Fry's Electronics to buy a camera surveillance system, where they meet an employee named Angel, who is obsessed with aliens and UFOs. They have Angel set up the cameras on top of the house and elsewhere on the ranch. Emerald steals a horse statue from Jupiter's Claim to use as a decoy, tying a pennant banner around it . That night, OJ investigates strange noises coming from the barn and sees three aliens. He punches one of them, revealing themselves to only be Ricky's children in costumes, telling him not to mess with Jupiter's Claim.
The UFO shows up and sucks up an escaped horse named Clover, as well as the decoy, but one of the cameras is blocked by a praying mantis. The next day, Emerald calls up Antlers Holst, a famed cinematographer who worked on the project the siblings got fired from. They ask for his help to record the UFO, but he declines, not wanting to encourage what he sees as a pursuit for fame. Angel shows up, having watched the camera footage from last night, and points out to the siblings a small lone cloud that never moves. They realize that that must be where the UFO is hiding. OJ starts to question if it's even a ship.
A flashback to 1998 shows Ricky on the set of Gordy's Home! during the chimpanzee incident. He hides underneath a table while the chimp attacks everyone else, which includes biting off the lips of teen actress Mary Jo Elliot. After the chimp calms down, it notices Ricky hiding under the table, and walks over to offer him a fist bump before getting shot in the head by animal control.
Ricky introduces the Star Lasso Experience, a new live show for his theme park. Among the crowd is Mary Jo, who now wears a veil to hide her face and rides in a motorized wheelchair. Ricky tells how for the past six months, he's been offering the Haywoods' horses to the UFO to gain its trust and plans to use Lucky to lure the UFO in an hour. The UFO arrives earlier than expected and consumes Ricky, his wife, his three kids and the entire audience, leaving Lucky alive.
OJ arrives to Jupiter's Claim to retrieve Lucky, but gets knocked unconscious when the UFO arrives. He wakes up hours later as it starts to rain. He calls Emerald to tell her that the UFO is not a ship, but a territorial, predatory animal just as it arrives at the house and cuts the power. The creature hovers above the Haywood house and spits out various objects it ingested such as the decoy horse statue and Mary Jo's wheelchair. This reveals it to be the one who killed Otis Sr.
The next morning, OJ leads Emerald and Angel to Angel's car, and realizes that the creature only attacks those who look at it. They hide out at Angel's apartment. Antlers learns of the 40 people who mysteriously disappeared at Jupiter's Claim on the news. When the Haywoods and Angel return to the ranch, Antlers is there and agrees to help. OJ decides to name the creature Jean Jacket after their horse.
To counter Jean Jacket's ability to shut off all electronics in its vicinity, Holst brings a hand-cranked IMAX film camera. They set up battery-powered tube men props across the ranch to track Jean Jacket's location in the sky. A TMZ reporter on a motorcycle with a reflective helmet and video camera shows up and asks Emerald what happened at Jupiter's Claim. He drives his motorcycle towards Jean Jacket's vicinity at 60 MPH, flying off it when the electricity in his bike turns off. OJ tries to save the reporter, but fails and the reporter gets eaten by Jean Jacket.
They manage to get footage of Jean Jacket, but Holst isn't satisfied. He climbs up a hill with his camera to get a better view, only to get consumed along with the camera. Jean Jacket tries to eat Angel, but he survives by wrapping himself in barbed wire and a tarp, causing Jean Jacket to spit him out. Now angry, Jean Jacket unfurls into its fully realized form, resembling a giant jellyfish with a green square for an eye. Emerald gets on her motorcycle, but Jean Jacket is too close. OJ looks directly at Jean Jacket to draw its attention away from Emerald.
Emerald drives her motorcycle to Jupiter's Claim and unties a large balloon mascot of Ricky, releasing it into the sky. She collects coins off of the ground and uses the Winkin' Well, an attraction that uses an analog camera to photograph whatever is directly above it, to take a photo of Jean Jacket. Jean Jacket notices the giant balloon and devours it before returning to its flying saucer form. The balloon bursts inside of Jean Jacket, killing it. Emerald screams out in victory as the photograph develops, capturing the fully formed Jean Jacket ensnaring the balloon. With the picture as proof of the creature's existence and reporters arriving nearby, Em sees an unharmed OJ and Lucky standing outside of Jupiter's Claim.
