Young-joo, who is focused on adjusting to her new job, grows tired of the constant complaints of her boyfriend, Jae-yoon, who's serving in the anti-aircraft unit at 77 A Tower. She breaks up with Jae-yoon, and he accepts the breakup with a heartfelt voice message. Upon hearing this, Young-joo heads to the tower to meet Jae-yoon, but on her way there, she encounters an unexpected incident.

Jae-yoon and his direct junior, In-ho, head down to Eden Hotel, right below their air defense unit, for an errand. Upon arrival, they find a plane crashing into the heart of Seoul, and the hotel turned into a bloody chaos. Meanwhile, Young-joo and Jin-wook also confront zombies and team up with Tae-shik, who happens to be passing by. Together, they begin their fight for survival.

Jae-yoon and In-ho, and Soo-jeong and Chef Jung from Eden Hotel manage to return to the air defense unit. However, even the base they thought was safe begins to face the threat of zombies. Meanwhile, Young-joo see the text messages sent by Jae-yoon and confirms that he is safe. She decides to go alone to Tower 77 A where Jae-yoon is located.

In the midst of the zombie crisis, Jae-yoon and Young-joo reminisce about their happy past and long for each other even more. Meanwhile, Jae-yoon and his squad mourn the loss of their comrades but find relief in the news of a supply helicopter's arrival. Young-joo’s group encounters Tripeater, who has been bitten by a zombie but has not yet turned.

After downing the supply helicopter, First Sergeant Jung Tae-hwa realizes that survival at their station is impossible. He plans to lead his platoon in an escape from the zombie-ridden A Tower. Meanwhile, Young-joo's group finally reaches the gate of A Tower, only to be greeted by a horde of zombies.

Young-joo is left by herself at the Express Bus Terminal Station, but a stroke of luck leads her to a metro staff office, where she gathers supplies to battle the zombies. Meanwhile, Jae-yoon's group reaches Eden Hotel, where they run into Arron Park, who has survived there alone. But the group gets split up by an unexpected attack by the zombies, throwing them into separate hotel rooms.

On her way to Suseo Station, Young-joo meets a very special person, who helps her reminisce on the good memories with Jae-yoon. She relentlessly fights the zombies alone on her way to Jae-yoon. Meanwhile, Jae-yoon and the squad successfully reach the 35th floor through the ducts, but they must find another way down to the first floor.

Jae-yoon escapes the A Tower first and saves others with a brilliant idea. But they must endure another goodbye. In the meantime, Young-joo runs into people who are even more brutal than zombies. Will Jae-yoon and Young-joo, the two lovers fighting their own separate battle, finally reunite in this chaotic apocalypse?