Thenali (Kamal Hassan) is a man who fears anything and everything. Dr. Panchabhootham (Delhi Ganesh) and his assistant (Ramesh Khanna), who are the doctors treating Thenali, are jealous of a relatively junior doctor Dr. Kailash (Jayaram) getting all the media attention. They decide to derail Dr. Kailash's success by sending Thenali over to him.
Soon after their first appointment, Dr. Kailash heads to his home in Kodaikanal on a vacation with his wife Jalaja (Devayani) and his two children. Dr. Kailash tells Thenali to wait until after the vacation before his therapy can begin, but Panchabhootham asks for Thenali to go meet Kailash while on vacation.
The expected happens. Thenali falls in love with Janaki (Jyothika), the younger sister of Dr. Kailash, which was vehemently opposed by the latter. The whole film is about how Dr. Kailash goes mad with the antics of Thenali and even begins to suspect that his wife has an affair with Thenali, as she has a soft corner for the latter. He even tries to kill Thenali by tying him to a tree with a time bomb, which Thenali (not knowing it is a bomb) removes and puts it in the doctor's house, causing it to explode and Dr. Kailash, seeing this, suffers a paralytic attack.
Evantually, Thenali marries Janaki and cures Dr. Kailash through shock treatment by bringing Meena, B. H. Abdul Hameed and K. S. Ravikumar and makes Meena act as his wife.