As the aftermath of the first film unfolds, Max (Patton Oswalt) grapples with a whirlwind of emotions, torn between feelings of resentment and an overwhelming instinct to protect Liam, the new addition to Katie's family (Ellie Kemper). His internal struggle reaches a peak during a mortifying trip to the veterinarian, where he is donned with an embarrassing cone. However, a twist of fate occurs when Duke (Eric Stonestreet) informs him that the family is going on a road trip, prompting Max to leave his worries behind.
Before they set off, Max leaves his cherished toy, Busy Bee, in the care of his devoted friend Gidget (Jenny Slate). Unfortunately, Gidget's kind intentions go awry as she accidentally loses Busy Bee in the home of a cat lady, infested with feline friends. Undeterred by the challenge, Gidget teams up with the skilled cat whisperer Chloe (Lake Bell) to sneak into the apartment and retrieve the beloved toy. With assistance from Norman, Gidget unexpectedly earns the title of "queen" among the cats, leading her to successfully reclaim Busy Bee.
Upon reaching Chuck's uncle's farm, Max feels disoriented by the new and unfamiliar landscape. A meeting with Rooster (Harrison Ford), a witty Welsh Sheepdog, serves as the jolt he needs to tackle his feelings about Liam and finally discard his dog cone. Although his initial attempts to mimic Rooster's herding prowess fail, Max is given the important task of finding Cotton, a missing lamb. This ultimately leads to a touching rescue mission that gains him Rooster's respect.
Meanwhile, the ambitious superhero wannabe Snowball (Kevin Hart) encounters Daisy (Tiffany Haddish), a lovely Shih Tzu who reveals her wish to rescue Hu, a majestic white tiger held captive by the treacherous Sergei (Nick Kroll). Together, they devise a daring plan to infiltrate Sergei's circus and free Hu. However, their mission takes a turn for the worse when they misplace Daisy's important flower clip, which alerts Sergei's pack of wolves to their presence.
As the story progresses, Snowball and Daisy find shelter at Pops' (Dana Carvey) apartment, where they must adapt to Hu's presence. Although Pops hesitantly accepts them, he quickly becomes overwhelmed by the ensuing chaos. With Sergei hot on their trail, Daisy and Snowball embark on a pulse-pounding adventure that eventually draws in Max and Duke, who join forces to thwart Sergei's wicked plans.
In an epic convergence of courage and teamwork, Gidget rallies her feline allies alongside Max's crew to outsmart the cat lady and track down Sergei's fleeing train. As the stakes escalate, this unlikely band of canines must unite their strengths to rescue their friends, confront challenges, and prove that even the most ordinary pets can become heroes.
As the train rolls on, Max seizes the moment to confront the pack of wolves terrorizing the passengers. At the same time, Snowball sends Sergei's pampered pet monkey into the fray, simultaneously liberating Daisy from her captivity and launching the monkey out of a cannon. With surprising accuracy, the monkey lands on Max's back, causing him to lose his balance and tumble off the moving train. However, undaunted by this setback, Max showcases his newfound bravery by leaping back aboard from the top of a tunnel, rejoining his companions in the pursuit of freedom.
Together, they endeavor to liberate Hu and vanquish Sergei from the train, putting their collective bravery to the ultimate test. Just when they think they can escape, Sergei reveals his true colors by brandishing a weapon. Fortunately, Gidget and her feline friends arrive just in the nick of time, using a well-aimed cat lady's car to subdue the villain and providing the relieved pets a safe route back home.
As peace is restored, Hu finds a new loving home with the gentle cat lady, while Max learns to alleviate his lingering worries about Liam, finally allowing himself to say goodbye as Liam embarks on his joyful new adventure in preschool.
