Firefighter Kim Ja-hong (Cha Tae-hyun) dies in the line of duty and is escorted to the afterlife by three guardians: Hae Won-maek (Ju Ji-hoon), Lee Deok-choon (Kim Hyang-gi) and their leader Gang-rim (Ha Jung-woo). They are to escort and defend him in seven trials in 49 days. If he passes, he will be reincarnated and his guardians will be given credit towards their own reincarnation.
In the first court, Hell of Murder, Ja-hong is tried for the indirect murder of his colleague, who was trapped beneath rubble during a fire. About to be sentenced, it is revealed that his colleague told him to prioritize the civilians. After rescuing 8 others, Ja-hong attempted to go back for his colleague only to be held back as the building collapsed. The judge then acquits Ja-hong.
In the 2nd court, Hell of Indolence, Ja-hong's guardians presented his selflessness and diligence. Ja-hong blurts out that he only worked for the money and was about to be punished when Gang-rim interjects, stating that the money was to support his brother and ailing mother and therefore acquitting him.
On the way to the Hell of Deceit, ghouls attack as the physics of Hell begins to morph. This change indicates that a member of Ja-hong's family had died and become a vengeful spirit. Gang-rim leaves the party to investigate as the rest proceeds. In the Hell of Deceit, Ja-hong is accused of writing fake letters to the family of those who had died, one of whom is Ji-yeon, the daughter of his fallen colleague. Gang-rim supernaturally links with Deok-choon from the living world to defend Ja-hong, stating that Ja-hong had also written fake letters to his mom to cheer her up, allowing her to focus on her health. And that the letters to Ji-Yeon, despite her actually understanding that her father was dead, helped her cope with her loss. The evidence is thrown out and the charges are dismissed.
Gang-rim determines that it is Ja-hong's brother Soo-hong who has become the vengeful spirit. He finds that Soo-hong has been accidentally killed while on guard duty by his partner Dong-yeon, and his death was covered up by their superior, Lieutenant Park. Gang-rim foils an attempt by Soo-hong to kill Dong-yeon and learns that the Soo-hong's grudge was not against Ja-hong but rather Dong-yeon and Lt Park. Dong-Yeon tips off Soo-hong's mother on the location of his body and attempts suicide. He is saved by Gang-Rim on Soo-hong's request after the latter agreed to cooperate.
The trio pass through the Hells of Injustice and Betrayal without trial. In the Hell of Violence, Ja-hong is tried for beating Soo-hong when they were younger while the latter was suffering from malnutrition. As Ja-hong was not forgiven, he is convicted and is about to be punished when Gang-Rim instructs Deok-choon to request for a combined trial at the next court, the Hell of Filial Impiety. The request is granted. Ja-hong reveals that, due to poverty, he intended to commit familicide but was discovered by Soo-hong. In a fit, he beat up Soo-hong and fled out of guilt, opting not to be a burden and dedicate his life to supporting them.
Gang-rim and Soo-hong arrive at the army base only to witness the mother's shabby treatment at the hands of Lt Park. Enraged, Soo-hong transforms back into a vengeful spirit and lashes out at the base, only stopping when he realizes how his actions are affecting Ja-hong in the afterlife.
At the Hell of Filial Impiety, Ja-hong is immediately judged by Yeomra to be guilty. He learns that on the night of his attempted murder-suicide, his mother was actually awake and had decided to allow it, knowing that she was a burden on her family. Soo-hong, with the help of Gang-rim and Won-maek, enters his mother's dream and learns that she had already forgiven Ja-hong for that night. Since sins that have been forgiven in the living world are not allowed to be judged in the afterlife, Ja-hong is allowed to reincarnate.
Afterward, Gang-rim realizes that Yeomra had been meddling with his investigation of Soo-hong. He decides to confront Yeomra directly and chooses Soo-hong as the group's final soul before their own reincarnation.