Arthur Brennan, an adjunct professor of physics, embarks on a journey to Aokigahara, often referred to as "The Suicide Forest," carrying a sealed package with him. Upon his arrival, he intends to take his own life through a drug overdose but unexpectedly encounters Takumi Nakamura, a Japanese man who previously tried to end his life by slitting his wrists after suffering a demotion. However, he now finds himself torn by memories of his wife Kiiro and daughter Fuyu, realizing he would miss them dearly. The two men soon find themselves lost in the forest, navigating through a thick maze of trees, and to keep their minds occupied, they share glimpses from their lives.
As flashbacks unfold, we learn about Arthur's troubled marriage to his wife Joan, a real estate agent. Their relationship is strained due to Joan’s resentment over Arthur's affair with Gabriella, his colleague, and the constant belittlement he faces for earning less than her. After a fierce argument leads Joan to suffer a heavy nosebleed, a hospital visit reveals she has a brain tumor. Remarkably, the couple manages to rekindle their love during this tumultuous period, reminiscing about their cherished lake house and the joy they found in the simple things, like walking among the orchids.
Tragedy strikes when a truck collision claims Joan's life after she is transported to a recovery hospital. At her funeral, Arthur confesses to the funeral director that despite their years together, he hardly knew his wife. The director reminds him about one of Joan's sisters sending a copy of her favorite book, triggering a wave of nostalgia.
In the forest, Arthur and Takumi face a series of harrowing challenges: falls, flash floods, dehydration, and the biting cold of the night. Amidst the despair, they come across the bodies of individuals who succumbed to their struggles. Taking refuge in a tent with a deceased camper's emergency supplies, Arthur shares with Takumi the deep remorse he feels for the way he treated Joan during their marriage.
The following morning, Arthur leaves a sick Takumi behind as he ventures out to seek help, promising to return. Using a walkie-talkie from the deceased camper, Arthur makes contact with park rangers, but his weakened state hampers his ability to convey that Takumi is still at risk in the forest.
Twelve days later, Arthur is assessed by a psychiatrist at the hospital before his anticipated release. He expresses his determination to return to the forest for Takumi, but the psychiatrist reveals troubling news: the rangers found the tent but no one else, and there are no records of a Takumi Nakamura or his family.
Returning to the forest, Arthur discovers the unopened package he previously abandoned. He finds the tent and a coat he used to cover Takumi but is unexpectedly greeted by a beautiful orchid in Takumi's place. He recalls Takumi's words about the forest acting as a purgatory, where the spirits of loved ones linger in moments of despair. Inside the package, he uncovers Joan's favorite book, Hansel and Gretel, leading him to a profound realization: Takumi was, in fact, a manifestation of Joan's spirit, guiding him to overcome his guilt.
Arthur ultimately returns to the United States, bringing the orchid with him. During a conversation with his student Eric, he discovers that the names Takumi mentioned were not identities, but rather words signifying "yellow" and "winter," leading him to remember his last meaningful exchange with Joan. He visits their lake house one last time, planting the orchid in Joan's garden, a symbol of love, remembrance, and healing.
