The No. 20 Madras Mail to Madras pulls out of Trivandrum Central with a curious mix of travelers, setting the stage for a night of intrigue and chance encounters. R. K. Nair, [M G Soman], a Kollam-based businessman, travels in the first-class coach with his wife Geetha, [Jayabharathi], and their daughter Devi, [Suchitra]. Also aboard are Tony Kurishingal, [Mohanlal], the son of a wealthy industrialist, along with his friends Hitchcock Kanjikkuzhi, [Maniyanpilla Raju], and Kumbalam Hari, [Jagadish], a young politician. The trio heads to Madras to witness a cricket test, enjoy a cabaret, and push Kanjikkuzhi’s detective novel toward a movie adaptation. All three are notorious drinkers, and their drunken bravado colors the journey. From Ernakulam Town, Sunil, [Ashokan], boards the same coach, quietly joining a tense, lively carload of characters. Later, film star Mammootty, [Mammootty], boards the train from Thrissur and finds the same compartment, turning an already crowded carriage into a further swirl of personalities.
As the train glides toward its destination, Tony and his companions try repeatedly to seat Tony in Devi’s coach, aided by the ticket inspector Nadar, [Innocent], but faces stiff resistance from the rest of the passengers, including Nair and Sunil. The tension rises when a quarrel erupts over seating, and the atmosphere lightens only when Mammootty’s arrival brings a momentary camaraderie. Devi’s presence sparks a mix of admiration and trouble, especially for Tony, who flirts with her as she returns from the restroom. The situation threatens to blow up until Mammootty steps in to diffuse the conflict, steering the trouble toward a quieter path.
Dawn reveals a grim truth: Devi is found murdered in the toilet at Madras, and suspicion instantly falls on Tony and his friends. The investigation is led by DySP Murthy, [V K Sreeraman], who collaborates with the new TTE, Chokalingam/Choki, played by [Jagathy Sreekumar], to piece together the circumstances. Choki reveals that Mammootty had taken quick photographs of Tony and his companions, a detail that becomes a key clue. Despite the gravity of the accusation, Mammootty uses his influence to help Tony and his friends return to Kerala, convinced of their innocence.
Back in Kerala, the mood shifts as Sunil confronts Tony, Hari, and Kanjikkuzhi at the lakeside, wielding a switchblade in a bid to uncover Devi’s killer. A fierce struggle follows, and Tony forces Sunil to reveal what he knows. Sunil confesses that he was Devi’s lover and that they had planned to elope from the train at Erode, but the escape failed when Sunil could not disembark in time during a halt. Devi had managed to board a taxi with the help of the station master and ride to Salem, rejoining the train. Sunil eventually reconciles with Tony and his friends, joining their circle as a new ally.
The key revelation unfolds when Sr. Gloria, [Sumalatha], explains that Devi left behind a diary, shared with her before leaving for Madras. The diary uncovers a troubling truth: Nair is not Devi’s father, but Devi’s stepfather, and he had killed her father in childhood. In a tense, orchestrated blackmail plan, Tony contacts Nair by impersonating Mammootty’s voice, demanding ₹20 lakh in exchange for the diary. Tony then coordinates with Mammootty to stage the exchange on the same train route. Mammootty boards the train at Palakkad, informing Nair that the diary won’t be handed over until Salem, where Tony will join the train.
Nair disembarks at Erode, while Tony, Mammootty, and Nair’s henchmen clash aboard a moving carriage. The showdown continues as Nair boards again at Salem Junction, only to be confronted by Tony. Under pressure, Nair confesses that Devi was killed when she confronted him about her father’s murder years earlier. By the time the train reaches Madras, DySP Murthy, having been briefed by Hari and Kanjikkuzhi, arrests Nair for Devi’s murder. Tony, Hari, and Kanjikkuzhi reclaim their innocence, restored by a chain of courage, wit, and a careful web of deception that exposed the killer.
In a final note of reassurance, on another journey back home aboard No. 19 Trivandrum Mail, Tony and his friends find Mammootty aboard again. This time, the meeting ends with a promise of a safe and happy return, and the trio heads home with a renewed sense of camaraderie and purpose, grateful for the turn of events that cleared their names and the strange night that brought them together.
