Prem Kumar (Vijay Sethupathi) is all set to get married to his lover Dhanalakshmi (Gayathrie Shankar). Two days before his wedding, Prem decides to try his hand at playing cricket with his friends Bagavathi alias Bugs (Bagavathi Perumal), Balaji Tharaneetharan alias Bhaji (Rajkumar) and Saras (Vigneshwaran Palanisamy). The game gets a little serious as Prem, in a bid to catch a ball, falls down, only to get injured in his head. He quickly gets back on his toes as the injury is not serious. Soon after the match, the friends are in for a shock. Due to the injury, Prem develops retrograde amnesia and short-term memory loss, wherein he remembers the chain of events which leads to the injury but forgets the last one year of his life and fails to remember everything after the injury. He remembers his life until college and recognizes his friends but is clueless about his upcoming wedding and about Dhana herself, as it is during the last one year that he met her, fell in love with her, and decided to marry her. Fearing trouble, his friends take him to a doctor, who says that he could recover anytime. Hence, his friends plan to keep him in their company, enact a drama until he ties the wedding knot, with the hope that he would get back his memory soon, while covering Prem’s disorder from everyone else. In the end, everything works out well, and Prem gets his memory back.

The marriage of Meera, a college student, and her boyfriend Saravanan has been approved by the family. But the condition is as after a job after graduation. But when Meera becomes pregnant, the whole story gets upside down

‘Seethakaathi’ is loosely based on the life of Seethakathi, a wealthy philanthropist, who had a passion for Tamil literature.