As 17-year-old Philip approaches the end of his high school journey, he clings to his aspiration of becoming a musician. His father, Frank, however, has a different roadmap laid out for him. With expectations set in stone, Frank insists on compliance with a summer internship at the family's ketchup factory and pursuing a state college education, all while dismissing Philip's dreams of attending NYU. One evening, Philip makes a heartfelt appeal to his mother, Laura, hoping to attend the Starling Festival at the lake to enjoy his last Spring Break as a high school student. Regrettably, despite her nostalgic memories of the festival, she denies his request, leaving Philip feeling despondent.
A twist of fate occurs when Philip receives an acceptance letter from NYU, but Frank's reaction is anything but warm. With no explanation or dialogue, he abruptly quashes Philip's dreams. In search of both escape and comfort, Philip makes the impulsive decision to sneak out the following day after Frank departs for a trip to Japan, while Laura and his sister Lib attend a PETA event. His loyal friend, Lewis, picks him up, and they head towards the lake.
However, the ideal vacation turns out to be less than expected as they find themselves at the modest home of Ross, Lewis' laid-back uncle, rather than a chic lakeside cabin. The day unfolds slowly as the boys lounge by the pool, and Philip's attention becomes fixated on his high school crush, Allison.
Just when they think the day is unremarkable, they spot Frank with a young waitress at the pool, leading Philip and Lewis to hide in a panic. Driven by curiosity, they follow Frank to her home, where they stumble upon an astonishing reality—Frank has another wife, Bonnie, along with children Kelly and Eddie. This earth-shattering revelation leaves Philip stunned and scrambling back to Ross' apartment to process this shocking family dynamic.
The next day brings further chaos. Philip, assuming the identity of Richie's son—a name that holds significant meaning for Frank—manages to gain a warm welcome from his newfound half-siblings. Upon Frank's unexpected return from Japan, the evening escalates dramatically as Philip exploits the situation, persuading his father to pay for his NYU tuition, resulting in a hefty check. As they spend time together, Philip creates bonds with Kelly and Eddie, sharing stories about their fathers without raising eyebrows—at least for now.
Meanwhile, Laura's concern about Philip’s extended absence prompts her to consult with Lewis' mother, who inadvertently reveals he has been by the lake. Realizing the urgency of the situation, Laura, along with Lib, decides to head to the lake themselves the following day.
Back at the lake, Philip and Frank receive timely warnings about Laura and Lib's impending arrival but are too wrapped up in their antics to heed the caution. Frank weaves an outlandish tale, justifying his absence from Japan, as the families prepare to rent a cabin for the festival, blissfully unaware of the brewing chaos.
On the morning of the event, Frank finds himself at Bonnie's cabin but feels an overwhelming urge to be elsewhere. He embarks on a rushed "run" to reach Laura's rental, leaving Eddie confused and alone. Upon arriving, Frank is met with disarray as Philip dispatches Lib and Laura to join him at the festival, unwittingly leading both families to converge at the same venue.
Just when it seems that everything might unravel, Ross appears with a powerboat, whisking Philip and Frank away to the festival in a flash, leaving the rest of the family to follow behind. As they split up to distract their wives, Philip's attempt to reach Laura is interrupted by Kelly, who confesses her feelings and tries to kiss him. Fortunately, Frank intervenes at just the right moment, but instead of letting the moment pass, he chooses to admit his infidelity. In a moment of chaos, Philip recalls a reckless comment Frank once made about risking his life; just then, a car comes barreling towards them. Frank instinctively pushes Philip out of harm's way, leading to both of them narrowly escaping injury yet landing in the hospital.
After the tumultuous incident, both father and son confront the emotional fallout. Philip sustains only minor injuries, while Frank is left with broken ribs and other wounds. Laura's reaction is icy; she leaves the hospital without confronting Frank, symbolizing a near-end to their marriage. Bonnie, along with their children, confronts Frank in a vulnerable moment, resulting in a profound sense of exhaustion for everyone involved.
Fast forward to months later—Philip is settling into his dorm room when Kelly appears, delivering the news that they will be living in adjacent rooms. This interaction highlights the unresolved feelings lingering between Philip and the complexities of his father's actions. In an emotional exchange, Frank visits Philip’s room and hands him his father’s guitar, a poignant gesture representing the relinquishing of his dreams. The guitar, which had never truly belonged to Frank, signifies Philip's newfound freedom to pursue his passion for music. As they embrace, a connection blooms between them—one that has been long overdue. Frank steps away, lingering for a moment outside Philip's room, as he listens intently to his son explore the strings, establishing a profound and resonant bond between them.
