We begin several years following the events of the previous adventure. A mysterious flying saucer departs from Witch Mountain, aiming for Los Angeles. Upon arrival, it lands in the bustling Rose Bowl Stadium, and its youthful passengers step out. Tony and Tia Malone, now teenagers, are dropped off by their guardian, Uncle Bene, for a much-deserved vacation in the city. They have spent the past years aiding in establishing their colony of extraterrestrial survivors while honing their remarkable supernatural abilities.
As they approach the secure front gate, Tony employs his telekinesis to navigate over it; however, Uncle Bene prompts Tia to simply unlock it instead. He sternly advises the siblings to reserve their powers for critical situations to avoid drawing unnecessary attention, urging them to relish their time away. They are soon picked up by Eddie, a self-absorbed taxi driver who boasts a flawless safety record, despite his reckless driving style. Regardless, the siblings find enjoyment in their ride through Los Angeles.
Meanwhile, a black car pulls into a secluded alley, and its occupants—a peculiar scientist, Doctor Victor Gannon, alongside his benefactor Letha Wedge and her conniving nephew Sickle—prepare to execute a diabolical plan. Gannon excitedly unveils his new mind-control device on Sickle, who has an intense fear of heights. Under Gannon's influence, Sickle reluctantly climbs to the roof of a tall building.
Unbeknownst to them, Tony and Tia’s taxi, driven by a distracted Eddie, begins to sputter. As Eddie disembarks to fetch gas, Tony experiences a vivid premonition of impending danger. He requests that Tia remain by the cab while he investigates, and with a sense of concern, she agrees.
Unfortunately, Gannon is thrilled with his successful experiment, and as Sickle finds himself precariously perched, Tia senses trouble. Racing against time, she arrives too late to intervene, witnessing Gannon and his accomplices capturing Tony and whisking him away in their vehicle. Henceforth, with Eddie still absent and the taxi now deserted, Tia takes it upon herself to scour the city for her brother.
Along her quest, Tia stumbles into a group of boys evading a tougher gang. They instruct her to run, and she follows, only to find herself cornered in an alley. Utilizing her telekinesis, she wards off their pursuers, allowing her and her new companions—dubbed the Earthquake Gang, composed of Rocky, Muscles, Crusher, and Dazzler—to escape. They commend her for her bravery and offer their assistance in finding Tony, recognizing her supernatural gifts but still being somewhat intimidated by them.
After a fruitless day of searching, the Earthquakes invite Tia to stay in their hideout, a decrepit attic in an obscure, abandoned house. As they plot their next move, Tia taps into her connection with Tony, receiving a brief vision that leads her to a dark void.
Meanwhile, Tony wakes up in Gannon's laboratory, bound on a table. A frantic Gannon prepares to unveil his mind-control device upon Tony, eager to wield his newfound power for influence and wealth. As Letha expresses her financial concerns over their operation, Gannon disregards her alarms. As Tony regains consciousness, Gannon swiftly doses him again, delaying any progress.
In the early morning hours, Gannon succeeds in implanting the mind-control device on Tony, activating it to demonstrate Tony’s incredible feats. Back with Tia and her crew, they continue to search for hospitals, baffled by the absence of their friend.
That night, Letha conspires to exploit Tony’s powers for a heist at a local museum showcasing gold bars. Tia, still frustrated that she cannot reach Tony, lies awake in her hideout, desperately trying to connect. The next day brings an opportunity that Letha is eager to exploit—Tony is instructed to create chaos at the exhibit to facilitate their escape, leading to utter pandemonium.
The Earthquakes, empowered by Tia’s sight, rush to the museum just in time to witness Letha and Sickle’s clumsy attempts at securing the gold. As chaos ensues inside, Tia’s determination surges. They sneak in just as Letha’s plan goes awry, and she and her minions realize that their heist might lead to a bigger mess than expected.
Finding her brother, Tia quickly devises a way to reverse the turmoil. Gannon, in a desperate attempt to regain control of the situation, orders Tony to eliminate Tia, not realizing the bond that exists between the siblings. Through sheer force of will, Tia breaks through the barriers of mind control, reminding Tony of their connection.
In the meantime, the unfolding crisis escalates, leading to a dangerous confrontation at a local plutonium plant where Gannon plans to leverage the threat of disaster for financial gain. Tia and the Earthquake Gang, having learned of the escalating danger, race to save Tony. Masterfully, Tia reclaims her brother, breaking Gannon’s grip, and together they assert their powers against their captors.
Finally, they send Gannon and Letha into the wait of authorities. In the end, Tia and Tony reunite with Uncle Bene, cherishing their adventurous holiday saga filled with supernatural abilities, friendship, and the importance of always standing together. As they prepare to board their ship home, Tony suggests a new kind of vacation—one that involves surfing.
