Family members gather at Mother's house to celebrate Monster's last birthday. Monster is the eldest son, but treated like an animal by the family since he is mentally and physically disabled. The family all agree to poison Monster on his birthday and keep it secret. As the family get drunk on food and drinks, their horrific backstories are revealed before Monster.
Lee Seung-won has worked in the film industry and theater since 2005. He made his first feature, Communication and Lies, in 2015, and Happy Bus Day is his second feature.