In the late 1940s fictional town of Oakton City, a purple squirrel named Surly (Will Arnett) and his mute rat partner Buddy reside in Liberty Park where their thieving reputation has made them outcasts. A group of Urban wildlife led by Raccoon (Liam Neeson) and his Cardinal assistant are running low on food for winter. Red squirrel Andie (Katherine Heigl) and glory hogging Eastern gray squirrel Grayson (Brendan Fraser) compete with Surly and Buddy to scavenge from a peanut cart manned by Lucky (Scott Yaphe) and Fingers (James Rankin) who are casing a bank. The squirrels' efforts inadvertently end with the cart's propane tank exploding in the park after its cord was bitten by Fingers' pug Precious (Maya Rudolph). The runaway cart ends up destroying and burning the tree, where the animals store their food. Surly is banished and Buddy follows him.
After his relationship with the other animals improved over the past nine months, Surly Squirrel is now loved by Liberty Park's Urban wildlife community, and currently overseeing an all-you-can-eat buffet at Maury's Nut Shop. This worries Andie, as she prefers a more hard-working outlook on life. Despite Surly's reassurances all is well, Maury's Nut Shop is accidentally blown up by Mole after he forgets to cut down pressure from the boiler. Andie takes the opportunity to try getting the animals back to their roots by foraging for food in Liberty Park while Surly and Buddy try to find other food-packed places, but fail each time. Defeated, he and Buddy decide to return to the park.