As Alvin, Simon, and Theodore Seville settle into their new home alongside their Chipette friends, Brittany, Jeanette, and Eleanor, they decide to throw a surprise birthday party for Dave. This event also serves as a good luck send-off for the Chipettes, who are about to guest judge on American Idol. The celebration draws an interesting crowd filled with various guests and celebrities, which does not sit well with Simon.
In the midst of the festivities, Dave takes everyone mini-golfing, where they meet Samantha, his girlfriend, along with her son, Miles. The Chipmunks make an effort to befriend Samantha but are quickly put on edge by Miles' hostile attitude towards them.
As the day progresses, the Chipmunks stumble upon a startling discovery: an engagement ring in a bag Dave brought home. They fear that he is planning to propose to Samantha and can't bear the idea of Miles becoming their stepbrother. In an attempt to prevent this, they devise a plan to retrieve the ring, but unfortunately, their efforts fall flat.
Complicating matters, Dave’s work obligations take him to Miami to produce a record for the rising pop star Ashley Grey, and he chooses to bring Samantha along. Determined to thwart the marriage proposal, the Chipmunks team up with Miles. To mislead their neighbor, Ms. Price, they come up with a quirky idea—dressing up three squirrels as stuffed animals.
Their adventure leads them on a flight, but mishaps ensue when Theodore’s antics prompt an emergency landing in Austin, Texas. This encounter draws the ire of Air Marshal James Suggs, who has a grudge against the Chipmunks, stemming from a breakup with his girlfriend on Christmas after she confessed to being their fan. In a bid for revenge, he places them on the No-Fly List (the only one exempted being Miles).
As they navigate through this chaos, the Chipmunks find themselves performing in a bar, where they are soon discovered by Suggs. Alvin steps into action, sparking a brawl that allows the group to escape quickly. They hop into a cab, only to be thrown out due to a lack of money. Exhausted, they find solace against an ancient tree, where Miles opens up about his troubled past of being abandoned by his father, which fosters a connection among the four.
While raising funds through busking, they successfully gather enough for a bus ride to New Orleans, where they participate in the vibrant Mardi Gras parade. This spectacle gains Dave’s attention as it gets broadcast live, and the Chipmunks use this opportunity to get back at Suggs, who becomes hilariously inebriated on moonshine.
When Dave and Samantha reach the Louis Armstrong New Orleans International Airport, they come face to face with a surprise reunion: Miles and the Chipmunks, fresh from their wild escapade. However, what follows is a scolding from Dave, who has been worried about their safety after being unable to reach them. In a twist, it is revealed that the Chipmunks made the No-Fly List, prompting Dave to take matters into his own hands and drive them to Miami.
Upon their arrival, a shocking truth comes to light: Alvin's earlier theft of the ring has inadvertently complicated Miles' proposal plans. As tension mounts, Miles feels the weight of the Chipmunks' indifference and comes dangerously close to a mishap on the streets of Miami. Fortunately, the quick-thinking Chipmunks intervene, using Theodore as a shield to save him from a precarious situation.
During dinner, Suggs enters their lives once more, only to find himself outsmarted and trapped in an elevator by the clever Chipmunks. This unexpected pause gives everyone space for reflection and reconciliation as they work to return the ring to Dave. In an act of goodwill, the Chipmunks extend a hand to Samantha and Miles, inviting them to be part of their unique family.
Yet, just when they think everything has settled down, Dave discloses that the ring belongs to his buddy Barry, who had planned to propose to Alice. The Chipmunks' mischief has unintentionally hindered Barry's romantic plans yet again. In a final attempt to make amends, they hit the stage at the launch party, joining forces with the Chipettes and Miles in a heartfelt performance to uplift Dave’s spirits.
As the drama calms, Barry seizes the moment to propose to Alice once more—and this time, she says yes! In a moment of happiness for all, Dave embraces the Chipmunks as his sons. However, this joy is short-lived when he discovers chaos caused by the squirrels in the house, leading to a mix of amusement and frustration.
Meanwhile, Suggs, released from his elevator trap, tries to unwind by the pool. His relaxation takes a sudden turn when security guards arrive to escort him away, reminding him that he's not exactly welcome as a guest.
