A young girl who goes through a difficult experience begins to see everyone's imaginary friends who have been left behind as their real-life friends have grown up.
Bea, a 12-year-old girl, moves into her grandmother's apartment in New York while her father undergoes heart surgery. She discovers a hidden world of imaginary friends, or IFs, and embarks on a journey to help them reunite with their original children, including herself. Through this magical adventure, Bea learns the power of imagination, the importance of family, and the healing nature of friendship.—Oscar Serpa
12-year-old Bea (Cailey Fleming) moves into her grandmother Margaret's (Fiona Shaw) apartment in New York while her father (John Krasinski) waits for heart surgery in the same hospital where her mother (Catharine Daddario) died of cancer years earlier. Even in hospital, Bea's father is a jovial person and finds moments to be happy by imagining hospital equipment like an IV stand to be his dancing partner. Janet (Liza Colon-Zayas) is a nurse at the hospital assigned to Bea's father.
While growing up, Bea had stayed with her grandmother on numerous occasions. Bea had a very active imagination as a child (that she got from her parents) and used to draw a lot, but she tells her grandmother that she is 12 now and she doesn't do that anymore.One night, Bea sees an unfamiliar creature, following it back to her grandmother's building to an apartment upstairs to her grandmother's. She hears a man's voice inside the apartment who refuses to open the door.
The next day, Bea sees it again, accompanied by a man. She follows them to a nearby house where the man, Cal (Ryan Reynolds), retrieves a large furry purple creature named Blue (Steve Carell). Blue was trying to connect with a little girl in her house, but she got scared looking at Blue and feel asleep after crying for an hour. Cal had come to take Blue back, while he insisted that he and the little girl were the perfect match.Bea also meets the other creature, a butterfly-like being named Blossom (Phoebe Waller-Bridge), and faints.
Bea awakens in Cal's apartment where she learns that he has been working with imaginary friends (or IFs) to place them with new children as their original children have grown up and forgotten them, and they will soon disappear. Initially reluctant, she eventually decides to help Cal. Blue indicates that he is excited that Bea "came back", but Cal interrupts him and Blue never completes what he meant.Blossom explains that IFs want to be with their kids, who have now forgotten them. Hence, they need new ones. Cal is not too keen to involve Bea, but she says she can help as she is a kid herself.
The next day, Cal takes Bea to Memory Lane Retirement Home, a retirement community for IFs housed underneath a swing ride in Coney Island.The IFs include Bubble (Awkwafina) bunch of bubbles that can pop and reconstitute themselves, Unicorn (Emily Blunt) with a pink and green mane, Spaceman (George Clooney), Ice (Bradley Cooper), Flower (Matt Damon) who resembles an anthropomorphic sunflower in a plaid suit, Banana (Bill Hader) who works as a water aerobics instructor, Art Teacher (Richard Jenkins), Slime (Keegan-Michael Key), Marshmallow (John Krasinski) whose head is always on fire, Octopuss (Blake Lively) a cat who wears an octopus costume, Viola (Allyson Seeger), Magician Mouse (Sebastian Maniscalco), Cosmo (Christopher Meloni) a detective IF with an obscured face, Ghost (Matthew Rhys), Guardian Dog (Sam Rockwell), Ally (Maya Rudolph) plush alligator, Gummy Bear (Amy Schumer) and Robot (Jon Stewart) an '80s-style toy robot IF in a domed helmet and one large wheel instead of legs.
An elderly teddy bear and the head of the facility named Lewis (Louis Gossett Jr.) inspires her to use her imagination to redesign the facility, much to the chagrin of Cal. Bea brings a lot of color and creativity to the drab looking retirement home (the rooms of each of IFs are redesigned to match their native look), only with the power of her thoughts.Bea tries to match one of the IFs with Benjamin (Alan Kim), a young patient at the hospital, but he is unable to see any of them. Lewis suggests to Bea that maybe IFs do not need new kids, but rather to reunite with their old ones.
Talking with her grandmother, Bea sees a photo of her as a young dancer and recognizes Blossom in the background. Realizing she was her grandmother's IF, she decides to test Lewis' idea. Playing one of her grandmother's records inspires Margaret to dance and she remembers Blossom, instilling Bea with hope.
Following a tip, Bea, Cal, and Blue find Blue's original kid Jeremy (Bobby Moynihan), now a grown man trying to launch a business. With Bea's help Jeremy remembers Blue, who gives him the confidence he needs for a business presentation. Turns out every adult has a trigger which makes them relive their old memories, which are associated with their IFs, and when those memories are triggered, the IFs automatically are remembered, and "come alive".
That evening, Bea arrives at Margaret's who frantically informs her granddaughter that there has been a complication with her father's treatment. Comforted by Cal, Bea says she does not want to say goodbye to her dad, so he suggests she tell him a story instead.
At the hospital, Bea tells her father a story about how she was pushing herself to act like a grown-up when she is just a child who still needs her father. He wakes up and they hug. When Bea goes outside his hospital room, she sees that all of the IFs, who had accompanied her to the hospital, are gone.
Returning to her grandmother's building, Bea goes to thank Cal only to discover that the door to his apartment opens into an old storage room as revealed by the landlady. After her dad is released from the hospital, he and Bea pack up to go home. During this, Bea realizes from an old picture she painted that Cal is actually her own IF, whom she had forgotten after her mother's death. Bea rushes to Cal's room and thanks him for helping, telling him she will always need him. This allows her to see Cal and the IFs again and they reunite one last time.
Sometime later, Cal continues reuniting the IFs with their original kids, now grown up. Benjamin meets his IF, a Cartoonish bespectacled dragon with a cast and crutch.