On his way home from a bar late at night, Do-joon sees a high school girl named Moon Ah-jung walking alone and follows her to an abandoned building.

The next morning, she is discovered dead on the rooftop, shocking the town and pressuring the incompetent police to find the killer.

When the mother questions the people in town about Ah-jung, they tell her the girl was sexually promiscuous and in a relationship with a boy known as Jong-pal, who had escaped a sanatorium.

The mother learns from a camera shop worker that Ah-jung had frequent nosebleeds and had pictures on her cellphone that she wanted to have printed.

The mother recognizes the man as a junk collector she once bought an umbrella from and goes to his home to find out what he saw, on the pretense of offering him charity medical services.

The police assume it got there during the murder, but the mother realizes that Jong-pal's story, that the blood is the result of Ah-jung's nose bleeding during consensual sex, is true.

Feeling guilty, she visits Jong-pal, who is even more intellectually disabled than her son, and cries for him when she hears he does not have a mother to fight for him, knowing he is going to jail for a crime he did not commit.

As the mother is about to depart from a bus station on a "Thank-You Parents" tour, Do-joon returns her acupuncture kit, which he found in the remains of the junk collector's house, and tells her to be more careful.

The site's critical consensus reads, "As fleshy as it is funny, Bong Joon-Ho's Mother straddles family drama, horror and comedy with a deft grasp of tone and plenty of eerie visuals.

[13] Manohla Dargis of The New York Times praised the performance by Kim Hye-ja and described the film as "alternately dazzling and frustrating".