Eight-year-old Pappu constantly troubles his mother Indira and frequently gets scoldings for his bed-wetting. His elder brother Kishore plans to watch a pornographic film with his friends in his room, and Pappu begs to join them. When Kishore refuses, he sends him away, and a fit of anger drives Pappu to cut the power, causing a short circuit at home. Indira scolds Pappu and shouts at her husband for not managing with just one child.
Distraught, Pappu leaves to end his life by the river, but a man stops him and takes him to his home. The man is a Scientist who runs his own laboratory. He convinces Pappu to take a medicine he invented that will transform a boy into a fully grown man. Pappu agrees and wakes up as a young man, though his mind remains that of a child. The only other person who knows the truth is Pappu’s classmate and friend, Deepak.
Now an adult, [Pappu], also known as Vichu, goes for an interview at a toy manufacturing company under the name Vichu. The owner believes a man who can empathize with children will understand their taste, so the job suits him perfectly. He meets the owner’s daughter Priya, whom Vichu had helped earlier at an amusement park, and the two soon fall in love. Their closeness irks Raj, another employee who admires Priya.
Vichu soon learns that his mother is depressed and sick after Pappu’s disappearance. After realizing her love for him, Vichu asks the Scientist to revert him to his eight-year-old self. The transformation happens, and Pappu reconciles with his mother. Yet at night, he reverts to Vichu, and he hurries back to the scientist, who is left unsure about what went wrong.
Pappu now lives as an eight-year-old by day and as a 28-year-old Vichu by night. Priya expresses her love for him, but he urges her to love a man more suitable for her age; nonetheless, he reciprocates her feelings after the scientist’s coercion. Priya asks him to accompany her to a matinee, but he declines so she won’t discover the truth of his condition.
Pappu’s mother befriends Priya as they become neighbors. Priya grows fond of Pappu but is frustrated by Vichu’s frequent absences. She resolves to marry him, and Raj abducts Vichu on the day of the wedding. During the day, he changes back to Pappu; the kidnappers release him, thinking they captured a child by mistake. That evening, Pappu becomes Vichu and marries Priya.
Priya longs for a child, and their union results in her becoming pregnant. Vichu/Pappu meets his mother and begins to understand the challenges of pregnancy, which makes him care for Priya with greater tenderness. Deepak demands ice cream, but Vichu refuses, citing his busy role as son, employee, and husband. The clash reveals to Priya that Vichu is Pappu, shocking her to realize she is carrying the child of a child.
Priya confronts Pappu about the revelations, and he reverts to Vichu. He explains that his love for her grew the moment he saw his mother in her. Priya eventually goes into labor and Vichu helps her reach the hospital. There, the truth about Pappu comes to light, and Priya gives birth to their son, though she remains distressed by the fact that she became the wife of a child.
Twenty years pass. Pappu is now a genuine 28-year-old who remains Priya’s husband. He returns to the room as a young man and emerges as the 48-year-old Vichu. Their son has grown to be a 20-year-old who bears a striking resemblance to him. Vichu reaches out to the Scientist to see if a cure has been found, but the scientist admits there is none.
