Rahul Mithaiwala is a 40-year-old orphaned bachelor living in Mumbai. Raised by his grandparents from the age of eight, Rahul’s grandfather Bhishambhar owns a chain of confectionery shops. Before Bhishambhar’s 100th-birthday celebration, Rahul’s friends Bobby and Baman invite him to accompany them on a vacation in Goa, which he accepts. Meanwhile, Bhishambhar dies on the eve of the celebration, just when he witnesses Sachin Tendulkar on TV in a cricket match get dismissed on his 99th run. After Bhishambhar’s death, Rahul’s grandmother Neetu tells Rahul that Bhishambhar wanted his ashes to be divided into two parts—one part to be immersed in the Ganga, and the other in Rameswaram. She asks Rahul to take the ashes to Rameswaram and disperse them. Reluctantly, he accepts her request but is also eager to attend the Goa trip.
Rahul, Bobby and Baman plan to dump the ashes at Goa but Neetu wants to see him off, forcing Rahul to travel by train. He books a single ticket on the Chennai Express, planning to meet Bobby and Baman along the way, at Kalyan Junction station. However, Rahul forgets to take the ashes, and he almost misses the train when he returns to collect them. As Rahul tries to leave the train, he notices a young woman running to board it. He helps her and four other people board the moving train, but the train leaves the station before he can get off.
Rahul tries to flirt with the woman, who starts communicating by singing parodies of Hindi film songs, and explains that the four men are trying to kidnap her. Rahul lends her his mobile phone so that she can contact her friends, but the men with her grab it and throw it from the train. Rahul is annoyed but says nothing because the men are carrying weapons. Rahul tells the Travelling Ticket Examiner about them, but they push the inspector into a river below a bridge. Panicked, Rahul learns that the four men are her cousins and that her real name is Meenalochni “Meenamma” Azhagusundaram. She is fleeing from a forced marriage to a ruffian named Tangaballi. Meenamma’s father, Durgeshwara “Durgesh” Azhagusundaram, is a powerful headman of the local mafia in Tamil Nadu.
Rahul (Shah Rukh Khan) helps Meenamma (Deepika Padukone) board the train, a spoof of the famous train scene in the 1995 film Dilwale Dulhania Le Jayenge
Meenamma takes Rahul to Durgesh and introduces Rahul as her lover. Tangaballi challenges Rahul to a duel that Rahul unknowingly accepts, due to his lack of knowledge in Tamil. On the night of the duel, Rahul escapes with the help of a local policeman, Shamsher, but he ends up on a ship with Sri Lankan terrorists and smugglers. A gun battle ensues between the police force and the terrorists. When the police officers investigate and take Rahul into custody, he tells his story and ends up back in Komban.
Terrified and once again surrounded by Durgesh’s sickle-wielding henchmen, Rahul pretends to take Meenamma hostage and escapes with her in Durgesh’s car, battling Durgesh’s men. When the car breaks down, Rahul and Meenamma quarrel and part ways. Rahul, not knowing which way to go, returns to Meenamma, who takes him to the Vidhamba village. Meenamma tells the villagers that they are a married couple who need protection and rest, to which the villagers agree.
Meenamma then realises that she has fallen in love with Rahul. When Rahul plans to sneak away, she argues, not wanting to earn the villagers’ distrust. Tangaballi catches Rahul as he tries to leave, but the villagers help them escape again. Meenamma persuades Rahul to disperse Bhishambhar’s ashes and travels with him to Rameswaram, where they complete the rites. On their way back, Rahul realises that he has fallen in love with Meenamma, and does not tell her where they are going. He takes Meenamma back to Durgesh and tries to make him understand and honour Meenamma’s wish. He also tells Meenamma that he loves her.
Rahul tells Tangaballi and his goons that he is ready for the fight this time. In the fight that follows, Rahul is severely injured but emerges victorious. Durgesh and Tangaballi reform, accepting that the love of a common man like Rahul is bigger than their physical ability and political influence. Durgesh allows Rahul to marry Meena. Rahul then leaves a message that love knows no regional or language barriers and that with a strong heart, there is no limit to what the common man can add.