Rahul Mithaiwala (Shahrukh Khan) is a forty-year old bachelor and lives in Mumbai. Rahul's parents died in a car accident when he was eight years old and it was his grandparents who brought him up. His grandfather has a sweet-selling chain store ? Y.Y. Mithaiwala. Before his birth centenary celebration, two of Rahul's friends suggest a vacation in Goa which he accepts. On the day of the birth centenary, his grandfather dies whilst seeing a cricket match. His grandmother tells him that his grandfather desired to have his ashes divided into two parts ? to be immersed in the Ganges River (in the north) and Rameswaram (in the south). She requests Rahul to go to Rameswaram and immerse the ashes. Rahul reluctantly accepts her request. On the other hand he was eager to attend the Goa trip, so, he and his friends make plans to dump the ashes at Goa. But, they were forced to change their plan when Rahul's grandmother told him that she would be coming to see him off at the station. This forces Rahul to travel by train and he booked a single ticket on the Chennai Express. He plans to meet his friends after commencing the journey at the Kalyan Junction station from where they would travel to Goa by car.
Rahul forgets to take his grandfather's ashes on reaching the station and when he comes back to get it, when the train was leaving the platform, he sees a girl running to catch the train. He helps her board the moving train. Then he notices four men also running to catch the train. Rahul helps them all to get in and himself misses the platform to get off. He learns that those four men were actually kidnapping the girl whose name is Meenalochni Azhagu Sundaram (Deepika Padukone) aka Meenamma. She was trying to flee from a forced marriage to Tangaballi (Nikitin Dheer). Her father Durgeshwara Azhagu Sundaram (Sathyaraj), is a powerful headman of the Komban Kaum (an anagram of Kumbakonam) in Tamil Nadu. He wants her to marry Thangaballi so that he would assume control of the neighbouring village as well. Meena takes Rahul to her father and introduces him as her lover. Thangaballi challenges Rahul for a duel which Rahul accepts in sign language. On the night of the fight, Rahul escapes with the help of a local Sikh policeman but in the turn of events, ends up in Komban Kaum again. Surrounded by sickle-wielding henchmen of Azhagu Sundaram, Rahul then pretends to take Meena as hostage and escapes again in her father's favourite car. When the jeep breaks down, they quarrel and go separate ways. But, Rahul not knowing which way to go, comes back to Meena who takes him to the Vidhamba village where a wedding was taking place. Meena lies to the people that they are a married couple and have eloped and needed protection and rest.
The villagers grow fond of the couple and take care of them as long as they stay while Meena realises that she has fallen in love with Rahul. When Rahul has plans of leaving secretly, Meena does not want to earn the distrust of the affectionate villagers. As he tries to leave, he is caught by Thangaballi but the villagers help him escape again. Meena who persuades him to immerse his grandpa's ashes travels with Rahul to Rameswaram. There they complete the rites and on their way back, Rahul, realising that he has also fallen in love with Meena, does not tell Meena where they go. He takes Meena back to her father again and tries to make him understand and honour his daughter's wish. He also confesses for the first time that he loves her too. Rahul tells Thangaballi and his goons that he has come ready for the fight this time. In the fight that follows, Rahul though severely injured emerges the winner. Meena's father and Thangaballi finally turn over a new leaf and accept that the heart of a common man like Rahul is much bigger than their physical ability and political clout. Azhagu Sundaram, allows Rahul to marry his daughter, the twist that changed Rahul's life forever. When they finally unite, the film leaves a message that love knows no barriers of region or language and that with a strong heart, there is no limit to what a common man can achieve.