Stéphane Miroux, played by Gael García Bernal, is a timid young man whose vivid dreams frequently disrupt his daily life. Following the death of his divorced father in Mexico, he relocates to Paris to live closer to his mother, Christine. Moving into his childhood home, he embarks on a new chapter by accepting a job at a calendar printing company, which his mother secured for him.
Anticipating a creative role, Stéphane eagerly shares his artwork—a series of twelve illustrations representing unique disasters, which he dubs "disasterology." However, to his dismay, his colleagues fail to appreciate his creativity; instead, he finds himself stuck in monotonous typesetting tasks that leave him disillusioned and frustrated, as emphasized in his vivid dreams.
One day, while on his way to work, Stéphane is warmly invited into the apartment of his new neighbor, Stéphanie. He soon discovers her artistic spirit, which encourages him. They embark on a collaborative project—a short animated film inspired by a boat that Stéphanie is constructing.
As the narrative unfolds, surreal and realistic elements start to intertwine in Stéphane's life, creating an intriguing ambiguity about what is real and what belongs to his dream world. One particularly whimsical dream sequence features Stéphane's hands transforming into enormous appendages, inspired by a recurring childhood nightmare of the film's director, Michel Gondry. As the division between dream and reality blurs, Stéphane finds himself falling for Stéphanie, sharing with her his imaginative creations, such as a 'one-second time machine'. Yet, this merging of realms causes him to question the authenticity of Stéphanie's feelings for him.
Stéphanie's whimsical toy horse, Golden The Pony Boy, echoes themes from The Outsiders, symbolically reflecting Stéphane's regression and childlike demeanor around her, likely stemming from the grief over his father's loss. He invents a mechanism for the toy horse, enabling it to gallop, and later receives a heartfelt call from Stéphanie, revealing that the pony was named in his honor.
To Stéphane's astonishment, the calendar company embraces his "disasterology" concept and throws a party in his honor. However, things take a turn when he succumbs to excess drinking after noticing Stéphanie dancing with another man. The following day, tension arises when Stéphane declares he no longer wants to maintain a friendship with her. Stéphanie proposes they address their issues over a date, but as he makes his way there, a vision leads him to believe she is absent. In a panic, he rushes to her apartment, only to find that she patiently awaits him at a café.
After this emotional turmoil, Stéphane decides it is time to return to Mexico. Before departing, his mother insists that he formally say goodbye to Stéphanie. In his awkward attempt, he inadvertently becomes rude, cracking inappropriate jokes. As his crassness drives her away, Stéphanie asks him to leave; nonetheless, he finds solace in her bed, discovering two significant items on her bedside: his one-second time machine and the completed boat meant for their animated venture. The film concludes with a poignant scene of Stéphane and Stéphanie joyfully riding Golden the Pony Boy across a verdant field before sailing off into the sea in her boat.
