Nallavan movie is all about Kocherukkan (Jayasurya) who is a happy youth and who lives an ordinary life in a quiet village somewhere in the border of Tamil Nadu and Kerala. He is in love with Malli (Mythili) who is an agile adventurous cheerful girl, who is ready to go to any extent to realize their love. Both of them are orphans, making a livelihood as servants in the house of two big landlords of the village.
As everyone knows about their love and their impending marriage, they are the pets of all in the village. The colourful life of the Kocherukkan and Malli takes a sea change, as something drastic happens, when they are just into their married life. And Kocherukkan is thrown behind bars for crimes that he has never even thought to commit.