Hawaiian siblings Pili and Ioane "E" have their roots in Brooklyn, New York. When Pili, an enthusiastic geocaching aficionado, triumphs in a competition that grants her free tuition to a summer camp nestled in the Catskills, her excitement is palpable. Unfortunately, their world is turned upside down when their mother, Leilani, decides that they must move to Oahu to assist their father, Kimo, who is grappling with serious health and financial challenges.
Feeling detached from her cherished life in New York, Pili finds solace in an old diary, liberated from her grandfather's art studio, which is conveniently set up in an abandoned bus. As she sneaks off the bus, she encounters local boy Casper, who provides her with a cover. That evening, while immersed in the diary's secrets, Ioane catches her in the act. He warns her about the Night Watchers, purportedly the spirits of valiant warriors that guard the island, trying to instill a bit of fear in her.
Meanwhile, E is distracted from his quest for better internet connectivity by local girl Hana, a friend of Casper. Hana expresses her distaste for E’s dismissive attitude towards younger children. In the diary, penned by the quartermaster Monks of the ship Peruvian, there are detailed accounts of a dangerous excursion led by Robinson and Brown, who had once mutinied, forsaking their ship and burying treasure on the island. Monks, left behind to guard the treasure while his comrades perished, entrusted the diary to Pili’s ancestors in exchange for their mercy.
When Kimo discovers Pili with the diary, his initial anger quickly dissipates as he decides to take her to see some landmarks referenced within its pages. Unfortunately, he suffers a fall that results in a broken rib. With their mother preoccupied at the hospital, Pili and Casper take the opportunity to embark on a treasure hunt, with E and Hana closely trailing behind.
The adventure takes them into the depths of a cave as they follow the diary's instructions. They navigate a narrow tunnel ominously dubbed the Jaws of Death, lined with stalactites, which elicits fear of a deadly spider from E. In a panic, he abandons the group and leaves behind the journal, now in Casper’s backpack. They soon find a precarious rope bridge over a chasm filled with lava, acknowledging it is too damaged to cross, yet the daring Pili and Casper bravely maneuver around it along a skinny ledge. E aids Hana in crossing, skillfully distracting her with a tune.
In a moment of vulnerability, E reveals to Casper that he was bitten by a brown violin spider, leading to a severe blister he is trying to hide with duct tape. As they make their way to a pool, they confront the next challenge: a tunnel submerged in rainwater. Pili cleverly rides on E's back to get through.
Arriving at what they call ground zero, Pili and Casper begin their search for the pirate treasure while E tries to persuade Hana to apply to Julliard, despite her reservations about losing her identity. Delving into the diary's complex pulley system, they realize that by shifting their weights onto a frayed rope, they can unveil the treasure room.
As they gather golden artifacts, Hana spots a religious statuette, provoking her to caution against disturbing what could be a tomb, fearing the wrath of the Night Watchers. As E dismisses her worry, they inadvertently awaken these spirits.
In a thrilling escape, they drop their loot and navigate their way through the canyon roots to safety, though E requires resuscitation when they reach the pools below. Leilani joins them shortly thereafter. Sheltering in a shed, Pili makes an offering with some of the treasures she had kept, and in a moment filled with reverence, they encounter the Night Watchers. Among them stands Kua, their father, who had died over a decade prior while serving.
In the aftermath of this emotional encounter, Leilani initiates a family meeting to deliberate whether they should remain in Hawaii or return to Brooklyn. The film concludes with joyful snapshots of their family enjoying life in Hawaii, while Ioane and Hana blossom into a couple, showcasing that adventure and family hold the deepest treasures of all.
