Synopsis:

Sometime after the events of the first film, Jack Russell Terrier Max's owner, Katie, meets and marries a man named Chuck. They later have a son, Liam, who Max disapproves of at first due to his rough play. Later, after Liam showed affection and adoration to Max and Newfoundland mix Duke (Max's roommate), Max's feelings soften up to when he becomes overprotective for Liam, even over him leaving for preschool. Later Max's overprotective feeling for Liam results in an itch which prompts Katie to get him a dog cone from the veterinarian in an effort to lower Max's symptoms. Max's luck changes when Duke reveals that the family is taking a road trip outside of the city. When Max's family reaches a farm, Max grows unaccustomed to the farm's ways, including a grumpy Welsh Sheepdog named Rooster. After an incident with the farm's sheep, Rooster, who is the farm dog, takes Max out to find one of the missing sheep that ran away. Rooster's teaching and encouragement to Max about acting unafraid prompts Max to successfully retrieve the sheep. Rooster allows Max to stay with him overnight as a result. Before departure, Rooster also gives Max one of his bandannas as a souvenir. Before Max left, he had entrusted his favorite toy, Busy Bee, to Pomeranian Gidget. Unfortunately, Gidget loses Busy Bee in a cat-infested apartment with one elderly lady known as the Crazy Cat Lady. She receives cat lessons from tabby cat Chloe, pug Mel, dachshund Buddy, and budgerigar Sweet Pea, so she can sneak in the cat apartment easier. With the help of guinea pig Norman, she successfully retrieves Busy Bee and uintentionally gets hailed as the "queen of cats after a staged defeat of the red laser dot." Meanwhile, Chloe and rabbit Snowball meet Shih Tzu Daisy, who is standing in their apartment who explains that she needs "Captain Snowball" (Snowball's daring persona) to assist her with rescuing a White tiger named Hu. While on flight to the city, Daisy encountered him being held captive by the abusive circus owner, Sergei. Daisy and Captain Snowball sneak in and find the circus Hu is in. With much difficulty from Sergei's black wolves, they free Hu from the circus. In the heat of the escape, Daisy accidentally drops her flower clip, which the wolves use to track her down. She and Captain Snowball take him to Basset Hound Pops' apartment. Pops reluctantly lets Hu live there at first, but due to his destruction of the apartment afterwards, Hu is kicked out and he, Daisy, and Captain Snowball hide in Max's apartment. Around then, Max's family returns from the trip and begin falling asleep, Max begins to fall asleep himself, but is woken by the trio. Sergei and his wolves track Daisy down and captures Hu. Daisy then sees him escaping in his train moving for the circus in which Captain Snowball, Max, and Norman pursue the train while contacting Gidget for assistance in the chase. Gidget and the cats take the Crazy Cat Lady off in her car, in pursuit of the train.