Mounam Pesiyadhe movie is about Gautham (Surya) who is a restaurant owner. He dislikes the idea of young couples flirting with each other in the name of "love". His close friend Kannan (Nandha), on the other hand, is just the opposite. He introduces himself as an employed bachelor to every girl he meets on the road.
Nanda falls in love with a young girl Maha (Maha). Unfortunately, Kannan's marriage is arranged with his cousin Sandhya (Trisha Krishnan). Neither Kannan nor Sandhya are interested in the proposal. When Gautham talks to Sandhya on behalf of Kannan and explains her that his friend is in love with someone else, Sandhya tells Gautham that she too is not interested in marrying Kannan. She even says that her dream is to marry someone like Gautham.
Gautham's disbelief in love fades out after meeting Sandhya. He falls in love with her. But, there is an interesting twist in the movie and it is not the usual climax you might expect to see.