Prashanth plays a rich kid preferring to stay away from home most of the day rather than endure his mother's constant shoutings. Being playful by nature, he manages to pull a fast one on Isha everytime he runs into her. Isha, daughter of an 'ideal' couple, has a rather poor opinion of Prashanth after these encounters.

On a trip to London to get away from it all, Prashanth leaves a poem at Diana's memorial. Isha, who is there as part of a Europe trip, sees the poem and immensely impressed by it, leaves another poem there. Their love for each other grows through more poems, all exchanged at the memorial. But circumstances force them to leave without meeting each other.

Back home, they still bicker when they run into each other while trying to find out who their unseen lovers are. Prashanth meanwhile is pursued by his secretary.

It's Isha who first finds out that Prashanth is the author of the poems. But when she tries to make him understand who she is, he misunderstands her feelings and rejects her leading to the climax.

The film follows Kaadhal Koattai's pacing with a boring first hour, then picking up speed leading to a well-executed climax. Prashanth does well but the movie's surprise is Isha. She's a revelation in this, her first movie and looks very pretty too. Kasturi prances about in micros and minis and has a song sequence more befitting a 'Silk' or a 'Anuradha'. Strong dialogs aid the director in many places and his skill can be seen in the way he manages to introduce tension into a completely predictable climax. Ilaiyaraja comes up with good tunes in Diana Diana... and Kaadhal Meedhu Kaadhal...