Calcutta Mail is a romantic thriller drama that revolves around a man named Avinash (Anil Kapoor), who travels to Calcutta for his son, but finds himself deluged, when others are interested in him. His only clue to find his son is a telephone number that was provided by a cop, before he came to Calcutta. In Calcutta, he meets Bulbul (Rani Mukherjee), a novelist who is doing research for her next novel. Due to circumstances, the two are compelled to share a room. Eventually Bulbul falls in love with Avinash, while he gets flashbacks of his wife Sanjana (Manisha Koirala) and their son. In the past, Avinash witnessed a crime and helped the victim. However, the attacker killed Sanjana and abducted their son. Since then, Avinash took matters in his hand, and his inquiries took him to Mumbai, where he might finally find whether his son is alive or dead.