Continuing the story of Max and his pet friends, following their secret lives after their owners leave them for work or school each day.
Max and Duke’s owner Katie marries a guy named Chuck, and the two have a son named Liam. Although Max is terrified of kids, he grows to love and protect Liam, but the horrors of the world (to a dog) are too much for Max, and it stresses him out. The family takes a vacation to Chuck’s uncle’s farm where Max and Duke meet a sheepdog named Rooster who shows Max that he doesn’t have to be afraid of the world around him.
Snowball now assumes a superhero identity and teams up with a Shih Tzu named Daisy to free an abused white tiger cub named Hu from a cruel circus owner named Sergei. He sends his wolves to find Hu.
Gidget is trusted by Max to look after his favorite toy while he is out, but she loses it in an apartment full of cats. With Chloe’s help, she disguises herself as a cat and gets into the apartment, but she wins over the cats when she “catches” the infamous red dot.
After returning home, Max finds that Snowball left Hu in Liam’s room. They get him out, but he is found and taken by Sergei, along with Daisy. Max and Snowball head to save them, and they are brought home by Gidget and the crazy cat lady that owns all the cats.
The cat lady adopts Hu, and Max and Duke go with Katie and Chuck to drop Liam off at his first day of preschool, where Max is confident that he will be okay.
Sometime after the events of the first film, Max and Duke’s owner Katie marries and has a child, Liam. At first, Max feels uneasy by his presence but eventually grows an affinity to him. Thinking of how dangerous the outdoors could be for Liam, Max develops an itch and gets a dog cone as treatment. Max’s luck changes when Duke reveals that the family is taking a road trip outside of the city.
When Max and Duke’s family reaches a farm, Max feels unaccustomed to the farm’s ways, including the farm’s sheepdog Rooster. After an incident with the farm’s sheep, Rooster 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 missing sheep. Rooster allows Max to stay with him overnight as a result. Before departure, Rooster also gives Max one of his handkerchiefs as a souvenir.
Before Max left, he had entrusted his favorite toy, Busy Bee, to his girlfriend, Gidget. Unfortunately, Gidget loses Busy Bee in a cat-infested apartment inhabited by a cat lady. After training received by Chloe, she sneaks in and successfully retrieves Busy Bee. Unintentionally, she gets hailed as the “queen of cats” after a staged defeat of the “red dot”.
Meanwhile, Chloe and Snowball meet Daisy, who is standing in their apartment who explains that she needs “Captain Snowball” (Snowball’s daring persona) to assist her with rescuing a tiger named Hu. —While on flight to the city, Daisy encountered him being held captive by the abusive circus owner, Sergei.— Daisy and Snowball sneak in and find Hu. With much difficulty from Sergei’s black wolves, they free Hu from the circus; however, in the heat of the escape, Daisy accidentally drops her flower clip, which the wolves use to track her down.
She and Snowball take him to Pops’ apartment. Pops reluctantly lets Hu stay for a night, but due to his destruction of the apartment afterwards, Hu is kicked out so they hide him in Max’s apartment. Around then, Max and Duke’s family returns from the trip. Sergei and his wolves track Daisy down and capture Hu. Daisy sees Sergei escaping in his train so Snowball, Max, and Norman chase the train down while contacting Gidget for assistance. Gidget and the cats take the cat lady’s car.
Snowball and Max defeat Sergei’s pet monkey and the wolves respectively, and free Daisy and Hu. Max, using his newfound bravery Rooster gave him, successfully infiltrates the train locomotive Sergei is in. The rest of the animals go and kick him[who?] out of the train. Before they are about to be killed by Sergei, Gidget, having teamed up the cat lady and her cats, knocks him out with her car and offers the animals a ride back home. Life resumes normally, with Liam’s family saying goodbye to Liam as he enters his first day of preschool.
In a mid-credits scene, Snowball creates a rap troupe, which only lasts a few moments before his owner, Molly, enters the room.
The film starts with Max (Patton Oswalt) and Duke (Eric Stonestreet) in Central Park where they watch other dogs playing with their owners’ kids. To Max, kids are a nightmare, and as another dog tells him, everything changes with a kid. Unfortunately for Max, his and Duke’s owner Katie (Ellie Kemper) meets Chuck (Pete Holmes), and the two eventually get married, move into Katie’s apartment, and have a kid of their own named Liam (Henry Lynch). Although Max feels tormented by Liam, he starts to warm up to the boy when he is old enough to walk and talk, telling Max that he loves him. From then on, Max makes it his job to protect Liam from the dangers of the world.
We see the other pets in their day-to-day activities. Chloe (Lake Bell) wakes up her owner by coughing up a hairball in her bed. Snowball (Kevin Hart) has now gotten used to being a pet to Molly (Kiely Renaud), and she dressed him up as a superhero, calling him Captain Snowball.
Katie notices that Max has been stressed out as he is constantly scratching himself when he thinks that Liam is in trouble. He has even gotten Buddy (Hannibal Buress), Mel (Bobby Moynihan), and Norman (Chris Renaud) to keep an eye on him throughout the city. As a result, Katie takes Max to the vet where he gets even more stressed out by the other anxious pets, and he has a cone placed on him to keep him from scratching.
The family plans a trip outside of the city to visit Chuck’s uncle’s farm. Before leaving, Max entrusts Gidget (Jenny Slate) in taking care of his favorite toy, a squeaky ball that looks like a bee. After Max leaves with the rest of the family, Gidget falls asleep with the ball and dreams of her and Max as being like a couple, but the ball slips from her grasp and it bounces all over the place until it makes its way down the fire escape and ends up in an apartment belonging to an old Cat Lady (Meredith Salenger) who lives with a LOT of cats, leaving Gidget too terrified to go in.
Snowball and Chloe are found by a Shih Tzu named Daisy (Tiffany Haddish), who needs help from Captain Snowball. He goes to change into his superhero outfit and is told by Daisy that while she was on an airplane with other pets, she found a scared and lonely white tiger cub named Hu that belonged to the circus. After getting off the plane, she noticed that Hu’s owner, a cruel circus ringleader named Sergei (Nick Kroll), was mistreating him. She wants Snowball to help her rescue Hu, and Snowball is more than ready to spring into action.
After arriving on the farm, Max and Duke try to get used to the new environment, but Max is chased by a wild turkey until it is stopped by Chuck’s uncle’s sheepdog Rooster (Harrison Ford). He comes off as harsh toward Max and Duke after Max drinks from his bowl.
Snowball and Daisy find the circus and come across Hu’s cage. However, he is guarded by four wolves that are chained up. Daisy manages to outsmart them and get three of them tangled up, but one comes loose and chases after Snowball through the circus until Daisy traps the wolf on a ride. She also gets the keys to the cage and frees Hu. The next morning, Sergei discovers that Hu is missing, and he finds Daisy’s flower that she left behind. He calls upon his wolves to bring Hu back.
Gidget goes to Chloe (who is high off catnip) for help on how to be a convincing cat to get Max’s toy back. Chloe puts her through “cat training”, which involves landing on her feet, using a litter box, and trying to devour Sweetpea (Tara Strong). Gidget almost actually does eat the bird until Chloe stops her.
On the farm, Max continues to stay by Liam’s side, even trying to read Little Red Riding Hood to him until they get to the part where the Big Bad Wolf eats Red and her grandma. Rooster sees how easily afraid Max is, and he even pulls the cone off his head. Later, Max accidentally lets the sheep loose from their pen until Rooster sends them all back inside, but a young lamb named Cotton (Sean Giambrone) is missing in the woods. Rooster makes Max join him in finding Cotton, and he is dangling over a branch that’s above a crevice. Rooster sends Max to get him free, which he does, but Max nearly falls to the bottom before Rooster pulls him up. That night, Rooster tells Max that he doesn’t have to be scared of the world, and the two howl at the moon together.
Snowball and Daisy try to find a place for Hu, so they bring him to Pops (Dana Carvey), who is looking after a bunch of puppies from around the neighborhood. Pops reluctantly agrees to keep Hu at the behest of the puppies, but they bring Hu back to Snowball after Hu trashes the place. Snowball then leaves Hu in Max’s apartment.
Gidget goes back to the cat apartment with Norman, using fake ears and a sock to make her look like a cat. She gets in and finds Max’s toy, but she awakens the big cat and is chased around by the other cats. Norman uses a laser pointer to distract the cats, and when they crowd around her, Gidget pretends to catch the laser dot and eat it. The other cats are amazed and they believe that Gidget is some kind of chosen one.
The family leaves the farm, and Max and Duke say goodbye to Rooster, who appears to be getting a little emotional (but he lets it pass quickly). Upon returning home, Max finds Hu inside Liam’s closet as the boy sleeps. Snowball comes to the room, and Max makes him take Hu out. Soon, they, plus Daisy and Gidget, see that the wolves are outside looking for Hu. They eventually corner Hu on the rooftop, where Sergei goes to tranquilize him and take the tiger with him, along with Daisy after she tries to intervene. The pets band together to go after them and save Hu and Daisy.
Max and Snowball ride to the circus train that everyone is on. After getting inside, Max is chased by the wolves while Snowball goes to get Daisy while dealing with Sergei’s pet monkey, Little Sergei. Max manages to get all the wolves to fall off the train, and Snowball and Daisy get Little Sergei trapped in a cannon. He gets shot out, but before Max can reach Hu, he gets knocked over by Little Sergei and they both fall off the train. Max runs to a bridge where he catches up with the train and breaks in. He, Snowball, and Daisy all attack Sergei before the conductor stops the train. They try to take Hu, but Sergei pulls on his chain and holds them all at gunpoint before Gidget arrives in a car driven by the Cat Lady and all her cats and knocks Sergei over. The other animals hop in before heading back to the city.
Cat Lady ends up adopting Hu. Snowball goes back to Molly where she now dresses him up as a princess, and he loves it. Max and Duke join Katie and Chuck as they escort Liam to his first day of preschool. Although Katie and Chuck are both sobbing messes, Max is now confident that Liam will be okay, and the boy hugs his two dogs goodbye for the day.
During the credits, Molly leaves Snowball with a panda doll. He then begins to perform the song “Panda”.