Twenty-six-year-old Ryohei Arisu is living a stable, peaceful life working as a school guidance counselor and is married to Yuzuha Usagi, the mountaineer he met during his first harrowing ordeal in the Borderland eight years earlier. When he learns that Usagi has gone into labor, he rushes back to Tokyo to be by her side. During his journey, a sudden, torrential rainstorm hits the city, and Arisu is struck and crushed by a falling billboard. This near-death experience immediately transports him back to the desolate, abandoned version of Tokyo known as the Borderland, a purgatorial dimension where people are forced to participate in deadly games to survive.

Arisu awakens in the eerie, empty streets with no clear memory of how he arrived, but his past experiences quickly come rushing back. He soon finds a small group of other players gathered at a themed building and learns they are all there because of the same accident. Their game is announced as the Nine of Hearts, titled Earth Invasion. Each player receives a phone that displays six LIFE cards and one DEATH card. To survive the simulated alien attacks, participants must enter a shelter after receiving a LIFE signal. The players initially cooperate, but the number of available shelters decreases each round, forcing them to make impossible choices about who will be left to die.

The new cast of characters includes a calculating doctor named Masatsugu Kitou, a volatile and violent man named Naomichi Taiga, and a wounded insurance worker named Yuzuru Horikawa who is haunted by his corrupt past. Also present are Yuzuru's daughter Himari and a woman named Nanase Onoue. As the game progresses, tensions rise and betrayals unfold. Kitou coldly proposes judging which lives are worth saving, while Taiga's violent nature erupts. Yuzuru volunteers to take everyone s DEATH cards in a selfless attempt to save the others, but his plan fails due to Kitou s secret manipulations. Himari eventually discovers that Kitou cheated, causing her father s injury, and retaliates by using her DEATH card on him.

Arisu uses his experience and tactical mind to uncover Kitou s strategy: he had been using a replica corpse s phone to avoid risk. Exposed, Kitou is forced to cooperate. Arisu devises a final plan to redirect all the DEATH cards to the corpse s phone and exchange LIFE cards among the surviving players. The strategy succeeds, and the game is cleared. The survivors awaken in a hospital in the real world, having survived their near-death experience. Arisu rushes to the maternity ward and arrives just in time to reunite with Usagi as she gives birth to their daughter, finally putting his second and final trip to the Borderland behind him.