Several items drift across the ocean, each carrying poignant messages from someone expressing feelings of hunger, boredom, and a desire to avoid dying alone. On a desolate island, Hank Thompson (Paul Dano) prepares to end his life, but his plans are interrupted when he discovers the lifeless body of a young man, Manny (Daniel Radcliffe), washed ashore. As Hank struggles to get to Manny, he accidentally tightens the noose around his neck, leading to a comically near-hang until the rope snaps.
In a moment of desperation, Hank checks for a pulse, only to find that Manny is dead. To his astonishment, a rumble emanates from the corpse, followed by a loud fart. Hank removes Manny's belt in another attempt at suicide, but the constant farting causes the body to shift, prompting a bizarre idea that leads Hank to utilize the gas for propulsion, reminiscent of a jet ski, only to tumble into the water shortly thereafter.
When Hank regains consciousness, he finds himself in a woodland area, convinced that Manny's body has miraculously saved him. He drags Manny into a cave to shield them from the downpour. As the night progresses, Hank is astonished to see Manny collect rainwater by cupping his mouth. The next morning, parched and desperate, Hank watches as Manny ejects the water from his mouth, which Hank eagerly drinks. Thanking the corpse, he begins to converse with it; to Hank's shock, Manny replies groggily with, "Okay, buddy." In a moment of surprise, Hank reacts aggressively, punching Manny.
Hank starts to see signs of life in Manny, who begins mumbling what sounds like "Manny," and it sticks. Although Hank doubts his sanity, starved and alone, he starts teaching Manny about the living world, from the basics of bodily functions to the intricacies of love. Amid their journey, they come across a sports magazine featuring a stunning woman, stirring something within Manny that leads him to develop a fascination with her. As they trek through the woods, Manny's newfound erections amusingly serve as a compass for their adventures.
Their trek is not without mishaps. Hank inadvertently steps in excrement, and then they hear a bear growling nearby, sending them tumbling down a hill. During the chaos, Manny stumbles upon Hank's phone, revealing a photo of the very woman who has captured his heart, Sarah (Mary Elizabeth Winstead). With newfound motivation to return home, Hank helps Manny understand love, even staging interactions where Manny awkwardly attempts to talk to Sarah through Hank's misdirection and roleplay.
As Hank builds several imaginative structures, including a bus and a theater, they indulge in playful scenarios where they explore their emotional lives, from discussing family dynamics to the realities of intimacy. With an inventive flair, Hank creatively uses Manny for various practical tasks, such as hunting and as a source for shadow puppetry performances of classic films like Jurassic Park and E.T.
Their adventure takes a perilous turn when they navigate a precarious pipe over a river, which collapses and sends them plunging into the water. With astonishing humor, they are propelled to the surface by Manny's flatulence yet again. Hank finds a brief moment of connection and hope when he gains service on his phone and sees a birthday message from his father, only to then face the heartbreaking reality of Sarah’s own life—she has a husband and a daughter. This revelation deeply wounds Manny, who feels misled about his chance at love.
As they hide in a tree to escape a bear, Hank sustains an injury as they attempt to draw attention to their perilous predicament. Just as despair sets in, Manny finds the courage to save Hank by lighting his fart on fire, startling the bear away. Following the ordeal, Hank awakens to find Manny carrying him toward the safety of a nearby house—it is Sarah's home. Their chaotic entrance leads to a frightened encounter with Sarah's young daughter, Chrissy (Antonia Ribero). The unusual sight of Manny causes a mix of confusion and terror, prompting Sarah to call for help.
Soon, paramedics, police, and reporters flood the scene, mistakenly thinking Hank is deceased when they spot Manny in a body bag. In a disheartening moment, Hank's father arrives, only to find the aftermath devastating. However, as Hank's chaotic tale unfolds, and his feelings for Sarah are displayed on social media, the bizarre nature of their adventure catches up with him. In a climax of frantic emotion, Hank carries Manny's body back to where their journey began, pleading for him to remain untouched as an officer attempts to apprehend him.
In a surprising twist of fate, as Hank’s friends and family watch on with mingled feelings of disbelief, Manny's farts propel him into action one last time, unleashing a string of absurd hilarity that draws smiles in an otherwise surreal moment.
