Zorg, a man in his thirties yearning to be a writer, sustains himself as a handyman for a cluster of beach houses nestled along the stunning Mediterranean coast in Gruissan, France. His life takes a tumultuous turn when he crosses paths with the 19-year-old Betty, a spirited and unpredictable young woman. They dive into an intense romance, living together in the modest shack he temporarily calls home. However, their relationship is anything but stable. Following an explosive argument, Betty violently destroys the shack, only to later discover Zorg's first novel manuscript. Captivated by his work, she declares him a genius.
Things take a dark turn when another disagreement prompts Betty to empty the shack’s contents and set it ablaze. The pair then relocates to the outskirts of Paris, seeking refuge at a small hotel owned by her friend, Lisa. As Betty fervently types Zorg's novel, submitting it to publishers, they encounter Eddy, Lisa's new boyfriend, and enjoy a series of wild adventures fueled by alcohol. Their carefree days lead them to work at Eddy's pizzeria, yet chaos isn't far behind when Betty stabs a customer during a tussle.
Despite Zorg's efforts to shield Betty from crushing rejection letters, she discovers one and violently confronts the publisher. Zorg steps in, threatening the man to avoid legal repercussions, voicing that Betty is his lifeline, the only good in his otherwise bleak life. When tragedy strikes with Eddy's mother's death, the group attends her funeral in Marvejols. There, Eddy proposes that they move into his late mother’s house and manage her piano shop. While Zorg relishes the peaceful rural life and forms bonds with quirky locals like Bob and his sexually frustrated wife Annie, Betty's erratic temperament poses a significant challenge.
One fateful day, an offhand remark from Zorg pushes Betty over the edge, leading her to shatter a window and spiral into a frenzied outburst through town. Hope seems to beckon when a home pregnancy test hints at impending motherhood, but disappointment soon follows with a negative lab result, plunging her into despair and causing her to hear disembodied voices.
In a desperate bid to secure Betty's happiness, Zorg devises a plan that leads him to rob an armored cash collection van while masquerading as a woman. His intent is noble, but chaos ensues when Betty, in her troubled state, impulsively abducts a young boy from his mother on a whim. As they flee the authorities, turmoil reigns.
Zorg's world collapses when he returns home one day to a macabre scene—blood everywhere and Betty missing. He learns from Bob about Betty's horrific act of self-mutilation, which has landed her in the hospital. Distraught, he races to her side only to find her heavily sedated. The following day, a phone call from a publisher brings unexpected good news, but his joy is short-lived. Upon his next visit, he discovers Betty in a catatonic state, requiring extensive treatment with no guarantee of recovery.
Fueled by anger, Zorg lashes out at the medical staff, believing her condition stems from her medication. After being forcibly removed from the hospital, he returns in disguise, whispering words of farewell before heartbreakingly suffocating Betty with a pillow. Alone once again, he sits down to write, conversing with his cat, whose presence seems to echo Betty's distant voice back to him.
