Max tries to hide from the child, but to no avail. However, he starts getting tormented by Liam when he starts talking and crawling. Max is confused by the new child, but realizes that he isn's a threat because he sleeps all day. She gives birth to a baby boy named Liam. A little while later, Max realizes that Katie is pregnant. Max thinks that he won't ever have to worry about a kid, but his owner, Katie, eventually meets a guy named Chuck. A beagle tells him that it's not fun, and once a kid comes into his and Duke's lives, they'll change. Duke tells him that having a kid seems like fun, but Max disagrees. The film starts out with Max narrating about how he is not a kid person. Max and Duke go home before Katie's return, and they are taken to the balcony and watch the scenery of Manhattan in the night together. Max mutters to Gidget about his appreciation, but he is interrupted by the other pets. Later, Tattoo drives a taxi and bring every animals home. Snowball jumps into the river and finds the key as it floats on the water, so he passes it to Max to unlock the cage. Max tries to reach the truck but he along with the truck fall into the river, and he is unable to unlock Duke's cage. The truck is thrown out of Brooklyn Bridge and hung above the river. At first, The Flushed Pets think Max attacks Snowball and decide to take revenge on him, but they are stopped by Gidget, who keeps fighting against the attacking animals. Therefore, Snowball and Max drive a bus to chase the truck and causes a huge chaos, and Snowball passes out after the bus crashes on the truck. Meanwhile, Snowball, with some The Flushed Pets members find Max, but Tattoo and Dragon are also captured. Suddenly, the Animal Control officers appear with a new truck and they capture Duke, so Max runs after it. Max encourages Duke to find his former master, Fred when they finally find Fred's house, they are told by Reginald that Fred passed away and the house belongs to another family.
Soon, they find a sausage factory, Weiner Kingdom, so they eat as much sausages as they can, but they are almost caught by the Animal Control officers. They end up escaping from Snowball through the sewer system which spits them out into the Hudson River - only for them to end up on a ferry to Brooklyn, in the opposite direction to home. In Snowball's lair, Snowball reveals the existence of other secret abandoned pets in the city and recruits the two to The Flushed Pets. However, they are rescued by a rabbit named Snowball, leader of a gang of abandoned pets, who frees them and crashes the van. Though the pair escape the confrontation by running away screaming, two Animal Control officers place them in metal cages for transit to the city pound. In an alleyway on the other side of Manhattan, Duke attempts to throw Max in a trash bin, only to meet a cat named Ozone and his gang of stray cats. They end up stuck in New York, far away from Manhattan, and are forced to work together to return home. Once out of sight, Duke forcibly drags Max out of the dog park and into downtown New York City. Later on, the duo are taken for a walk by a dog walker to the dog park in Washington Square Park, where Max's insults make Duke very angry, which leads him to tricking Max away from the others to get him alone. When Duke destroys a vase in the living space and spills dog food in the kitchen area, Max attempts to frame him by making more of a mess, knowing that Katie will assume Duke is the culprit. The next day, Max decides to displace Duke and become the object of Katie's affections once more. Although Max is immediately disgusted by Duke's lack of boundaries and selfish sleeping habits, his attempts to warn Katie about Duke are in vain as she's unable to understand his barking. One day, she comes home with another dog, a mutt she adopted named Duke. He spends his days waiting by the front door for Katie to return whenever she leaves and playing with his friends Mel and Buddy. In Manhattan, Katie found Max as an abandoned puppy in a cardboard box and adopted him as her pet, then took him back to her apartment in New York, where Max still lives happily with her.