In an unexpected twist that shakes up the Pickles family, Didi welcomes her second child, Dil, into their lives. As the family adjusts to this new arrival, Tommy, feeling uncertain, worries about how this will change his relationship with his family. However, Stu reassures him that as time passes, he will learn to cherish having a little brother like Dil at his side. Nevertheless, Tommy finds it challenging to connect with his rambunctious sibling, who appears more interested in instigating chaos rather than forming a bond.
As tensions escalate at home, Phil and Lil suggest a daring plan to ease the situation by utilizing Stu's Reptar Wagon to take Dil back to the hospital. However, their scheme derails rapidly when Angelica enters, demanding silence from the boisterous group. In the middle of the chaos, Dil snatches Angelica's cherished Cynthia doll, igniting an explosive confrontation between them. Just then, the Reptar Wagon unexpectedly launches into action, speeding through the streets with the babies aboard until it careens into a secluded wooded area.
As their beloved pets and belongings scatter in disarray, Angelica leaps into action to retrieve her prized doll. She commandeers the family dog, Spike, and ventures into the woods, leaving the Pickles family in a state of panic. Meanwhile, Drew and Charlotte arrive to find the media swarming the neighborhood, sensationalizing the mysterious disappearance of Tommy's friends.
Amidst the search operation sweeping through the woods, Tommy takes it upon himself to rescue his stolen brother, Dil, who has unwittingly gotten into trouble with a group of mischievous monkeys rummaging through a wrecked circus train. Ignoring his friends' hesitance to help, Tommy embarks on a daring solo mission, fueled by determination to reunite with his brother and put an end to their whirlwind adventure.
As a fierce storm rages on, Tommy’s frustration boils as he desperately tries to manage his wayward brother Dil. Despite Tommy's best attempts to bond with Dil, the little one remains fixated on his own whims. Driven to his limits, Tommy considers dumping some Mash Banana Baby food on Dil, hoping to attract the monkeys' attention and restore peace. However, the storm provokes Dil's tears, and, surprisingly, it catalyzes a change, allowing him to connect with Tommy as they face the storm together.
With the rain finally easing, the brothers reunite with Phil, Lil, and Chuckie, who have experienced a change of heart and prevent the monkeys from reemerging to seize Tommy and Dil. Meanwhile, Angelica successfully recovers her beloved Cynthia doll and rejoins the group. The reunited babies then embark on a thrilling trek over a damaged bridge, only for Angelica to lose her grip and dangle perilously over the surging river below. In a tense standoff against the monkeys, a massive wolf emerges, threatening the babies until Spike makes a heroic entrance, bravely defending them at the cost of his own life.
As the turmoil settles, Stu, piloting a pterodactyl-inspired glider, spots the babies from above and crash-lands into a nearby ranger's cabin. Mistakenly believing he is the "lizard," the babies plead with him to fetch Spike's body instead of returning home. However, as Stu tumbles through the bridge's supports, he uncovers Spike's miraculous survival. Eventually, the children are joyfully reunited with their parents and return home, marking the conclusion of their gripping and tumultuous adventure.
