It's a story about lucky love and how love stays strong. The title is formed from the first alphabets of the lead pair's name in the film: Jagan and Jamuna. Jay Jay is a remake of the Hollywood film Serendipity, starring John Cusack and Kate Beckinsale.

Jagan -Madhavan and Jamuna -Amoga meet in a shopping mall in Chennai. They like each other and spend a day chatting in the coffee shop. Jamuna, a believer in destiny, writes down her address and telephone number on a Rs.100 note and makes the payment at the cafe declaring that if the note comes back to Jagan then it means that he was destined to meet her. He tries hard to trace the note but in vain. Jamuna goes back to Kolkata.  Meanwhile Pooja -Pooja, who was rescued by Madhavan from some thugs, has a crush on him. Finally Jagan manages to get Jamuna address on the eve of her wedding and unites with her.