Chachi 420 is a romantic comedy drama. The movie revolves around a man named Jaiprakash Paswan (Kamal Hassan), who lives with his wife Janki (Tabu) and daughter Bharti (Baby Sana). Janki is daughter of a rich businessman, while Jaiprakash is an assistant director in films. The two get married against the will of Janki’s father, however, after marriage Janki couldn’t get used to her middle-class lifestyle and also other differences come between the couple, so she files for a divorce and takes her daughter with her and doesn’t allow Jai to meet her. Soon after, Jaiprakash disguises himself as an old lady and gets the job of a caretaker of his daughter, in order to spend more time with her. However, what will happen when Janki will learn the truth?