In a serene grassy landscape, a water buffalo manages to break free from its tether and wanders into the depths of a forest, where it catches the attention of a man wielding a sickle. Unbeknownst to them, a shadowy figure with red eyes observes from afar.
In a quaint farmhouse, Boonmee lives with his sister-in-law Jen and his nephew Tong. Boonmee is grappling with his declining health due to a failing kidney, receiving dialysis treatments from his Laotian assistant Jaai. One evening, during a family dinner, the spirit of Boonmee's late wife, Huay, unexpectedly appears. Having passed away over ten years ago, Huay reveals that she has sensed their prayers and is aware of Boonmee's deteriorating condition. Shortly after, a wild figure with hairy red eyes emerges, identifying himself as Boonmee's long-lost son, Boonsong. Boonsong, who had vanished some time after Huay’s death while pursuing his passion for photography, recounts his encounter with a supernatural being he refers to as a "Monkey Ghost." He shares that he engaged with this creature, a fusion that drastically altered his appearance, leading him to forget his prior life.
As the day unfolds on the farm with Jen, Boonmee reflects upon his affliction, attributing it to karma due to his past actions—specifically, the communists he killed during his military service and the insects he dispatched on the farm.
Meanwhile, a princess is being carried through the forest in a litter. Upon reaching a waterfall, she gazes into its waters, finding her reflection more radiant than her actual self. After a brief kiss from a servant, she claims he merely imagined it. In her solitude, she is enchanted by a catfish’s compliment and seeks to exchange her jewelry for the beauty of her reflection, leading to an intimate encounter.
In a tender moment, Boonmee is found lying next to Huay. He embraces her and inquires about reuniting with her after death. Huay explains that spirits are bound to individuals rather than locations. Later, Boonmee, Huay, Jen, and Tong explore the forest where they glimpse enigmatic figures darting through the trees. Intrigued, Huay guides them into a cave, where Boonmee shares a fragmented memory of being born there, although he can’t fully recall it. He dreams of a future world where authorities eliminate “past people” with light. A significant moment arises when Huay detaches Boonmee's dialysis tube, leading to his tragic passing by the next day.
After the somber funeral, Jen is seen sorting through gifts of baht with her friend Roong. Meanwhile, Tong, who has taken vows as a monk, arrives but struggles to find peace in the temple. After a quick change from his robes to casual attire, he and Jen prepare for a meal. In a surreal twist, Tong is startled to see himself alongside Jen and Roong relaxing on the bed, while they continue their mundane evening, caught between two spaces.
