As the story unfolds, we are welcomed into a charming world through a heartwarming montage that showcases the loving memories of a family, particularly from the perspective of young Bea (Cailey Fleming). She delights in playtime with her parents, Dad (John Krasinski) and Bea's Mom (Catharine Daddario), painting a picture of idyllic childhood bliss. However, an underlying shadow looms as cancer slowly begins to affect their family dynamic, forcing Bea's mother to maintain a strong façade for her daughter's well-being. Despite her courageous attempts to stay strong, sadly, she ultimately succumbs to the illness.
After the heartbreaking loss of her mother, a resilient Bea, now 12 years old, finds herself moving in with her grandmother Margaret (Fiona Shaw) while her father prepares for an upcoming heart surgery. He crafts imaginary friends in an effort to lift spirits, but Bea holds her feelings tight, opting for stoicism over vulnerability. One fateful day, while visiting her father, she befriends Benjamin (Alan Kim), a boy prone to mishaps, who brings a much-needed comic relief to her life.
The night takes an unexpected turn when Bea ventures out to find a charger for her mother’s camcorder, only to encounter a mysterious creature that defies explanation. Mistaking it for another child, her approach leads to a shocking realization that this being is not alone—Cal (Ryan Reynolds), a somewhat gruff yet endearing fellow, is attempting to convince her there's no one there. Yet, a flicker of belief within Bea senses otherwise.
The next evening, Bea observes Cal with this strange entity before disappearing through another apartment door. Here, she witnesses Cal entering the bedroom of a little girl while trying to coax out Blue (Steve Carell), an imaginative friend ready for his new companion. Unfortunately, Cal's efforts result in waking the child, forcing him to hide beneath her bed. Upon emergence, he meets Bea, whose initial panic soon dissipates when she encounters Blossom (Phoebe Waller-Bridge), an unorthodox bug-like imaginary friend who attempts to make her acquaintance. Just as reality twists, Bea faints before this bewildering sight.
When she awakens in Cal's realm, she is surrounded by an unexpected trio: Blue and Blossom explain they are not mere companions, but imaginary friends (IFs) on a quest to form a matchmaking agency to connect with new childtainers. Their existence hangs in the balance, as fading into obscurity is their imminent fate without this connection. Thus ignited, Bea embraces her sense of purpose to aid them.
Her hospital visit takes a surreal turn when Blue appears beside her father's bed. Mindful of the implications, Bea urges Blue to vanish, fearing repercussions from those unacquainted with the imaginary. Yet, her fate leads her to a Coney Island adventure with Cal, revealing a hidden realm of IFs during their escapade.
Inside this secretive passage, Bea meets an array of quirky IFs, including Flower ([Matt Damon]), a nurturing sunflower guide, Guardian Dog ([Sam Rockwell]), a heroic canine, and more extraordinary characters. The arrival of Art Teacher ([Richard Jenkins]) connects her to Lewis ([Louis Gossett Jr.]), a wise teddy bear IF who holds the key to rekindling forgotten memories.
As she grapples with this bizarre fantastical reality, Bea stumbles upon a surreal performance reminiscent of a Tina Turner musical, deepening her connection to these magical beings as she navigates her unique journey through Art Teacher’s paintings. With Lewis encouraging her to allow memories to wash over her, Bea unlocks a simultaneous recognition across a vibrant Coney Island landscape, establishing her intent to aid the IFs in their quests while discovering her own identity within the narrative.
As Bea dedicates herself to connecting Benjamin with an Ideal Form, her journey introduces her to an imaginative cast of characters, each claiming to be his perfect counterpart, including Uni ([Emily Blunt]), the bubbly unicorn, and Marshmallow Man ([John Krasinski]) - embodying an extravagant charm. Despite their colorful personas, none can become visible to Benjamin, further complicating her goal.
Digging into her mother's past reveals hidden depths, including her history as a dancer. A photograph surfaces showing Blossom as her mother’s former Ideal Form, which prompts Bea to connect with Blossom, although met with hesitation. Undaunted, she enlivens the situation by spinning a vintage record, which unexpectedly awakens her mother's latent dance potential, inspiring Blossom as they find their common rhythm.
Meanwhile, with Cosmo uncovering insights about Jeremy ([Bobby Moynihan]), Blue’s estranged son, Bea, Cal, and Cosmo delve into Jeremy's life, leading to poignant revelations about reconciling with one's past. As Bea returns home, a heartwarming welcome awaits her, reinforced by her father’s recovery at the hospital and the loyal support of her whimsical companions.
In a twist of fate, as Margaret hopes for a meaningful reunion with her family, the mystical adventure transcends into everyday life, showing growth for both Bea and her imaginary entourage. Following the credits, Lewis makes a brief reappearance, offering a touching culmination dedicated to the cherished memory of Louis Gossett Jr., carving a bittersweet yet hopeful end to this enchanting tale of love, loss, and the enduring magic of imagination.
