Riley Andersen is born in Minnesota. Within her mind's Headquarters, five personifications of her basic emotions — Joy, Sadness, Fear, Disgust, and Anger — come to life and influence her actions via a control console. As she grows up, her experiences become memories, stored in colored orbs, which are sent into long-term memory each night. Her five most important "core memories" (all happy ones) are housed in a hub; each powers an aspect of her personality which takes the form of floating islands. Joy acts as a de facto leader, and since she and the other emotions do not understand Sadness' purpose, she tries to keep Sadness away from the console. At the age of 11, Riley and her parents move to San Francisco for her father's new business. Riley has poor first experiences: the new house is cramped and old, the only pizza is pizza topped with broccoli, her father is under stress from his business, and the moving van with their belongings won't arrive for weeks. When Sadness begins touching Riley's happy memories, turning them sad, Joy tries to guard them by isolating her. On Riley's first day at her new school, Sadness accidentally causes Riley to cry in front of her class, creating a sad core memory. Joy, panicking, tries to dispose of it, but accidentally knocks the other core memories loose during a struggle with Sadness, deactivating the personality islands. Joy, Sadness, and the core memories are sucked out of Headquarters and taken to the maze-like storage area of long-term memory. Anger, Fear, and Disgust try to maintain Riley's happiness in Joy's absence with disastrous results, distancing her from her parents, friends, and hobbies. As a result, her personality islands gradually crumble and fall, one by one, into the "Memory Dump", an abyss where memories are forgotten. Finally, Anger inserts an idea into the console, prompting Riley to run away to Minnesota. While navigating through the long-term memory region, Joy and Sadness encounter Bing Bong, Riley's childhood imaginary friend, who suggests riding the train of thought back to Headquarters. En route to the train station, Bing Bong tearfully watches his rainbow wagon rocket being thrown into the memory dump along with other unused childish artifacts. The three eventually catch the train, but it halts when Riley falls asleep, then derails entirely when "Honesty Island" collapses due to Riley's theft of her mother's credit card. In desperation, Joy abandons Sadness and tries to ride a "recall tube" back to Headquarters, but the ground below the tube collapses, breaking it and plunging Joy and Bing Bong into the Memory Dump. At the bottom of the abyss, Joy begins to lose hope and breaks into tears, but then discovers a sad memory of an ice hockey game that turned happy when Riley's parents and friends comforted her. Joy finally understands Sadness's purpose: to induce empathy in others, prompting them to reach out to Riley when she is emotionally overwhelmed and needs help, and by preventing her from feeling sad, she was also preventing her from feeling true happiness. Joy and Bing Bong try to use the wagon rocket to escape the Memory Dump. After two failed attempts, Bing Bong, who is already fading away, jumps out to allow Joy to escape and is forgotten. Joy reunites with a despondent Sadness and takes them to Headquarters, only to discover that Anger's idea has disabled the console, rendering Riley apathetic. To the surprise of the others, Joy hands control of the console to Sadness, who is able to extract the idea, reactivating the console and prompting Riley to return home. As Sadness re-installs the core memories, turning them sad, Riley arrives home to her parents and tearfully confesses that she misses Minnesota and her old life. Her parents comfort her and admit they, too, miss Minnesota as much as she does. Joy and Sadness work the console together, creating a new amalgamated bittersweet core memory in Riley's Headquarters; a new island forms, representing Riley's acceptance of her new life in San Francisco. A year later at the age of 12, Riley has adapted to her new home, made new friends, and returned to her old hobbies while adopting a few new ones. Inside the Headquarters, her emotions all work together on a newly expanded console with room for them all.
