Mahasamudram movie is about Isahak (Mohanlal) who is a fisherman with the heart in the right place. He is the pivot around which an old-age home run by a priest (Jagathy Sreekumar) revolves. Isahak bears the entire expenses of the home, one of whose inmates is his mentally ill father Velankani (Innocent). Incidentally, Isahak is also the star player of his football club called ‘Beach Eleven’ of Puthenkara. Their archrivals belonging to village Mattukara are named Seven Stars. Of course, Mattukara is under the control of the baddies like Riza Bava.
Isahak falls in love with Riza Bava’s sister Devi (Laila). The brother is not amused, and he vows to frustrate the lovers’ plans. the lovers get married on the high seas and plan to spend their first night on the boat. The bride’s brother is bent on killing the newly-wed couple.
But fate has other ideas. Devi accidentally kills her brother. She is arrested, and a heart-broken Isahak is left to fend for himself. Meanwhile, Devi comes out of jail on parole. He is caught between his wife and father. How does he resolve it is what the remaining story is about.