Arunam movie tells the story of the trials and tribulations that a person who is an out and out idealist has to face in life. Achuthan (Manoj K Jayan) who is a factory worker, a trade union leader and a sworn communist. Commitment to the party takes priority over even his family. He is also an ardent lover of music. His wife is Seetha (Geethu Mohandas).
Munisamy (Mala Aravindhan) who is a co-worker in the same factory. He hails from Tamil Nadu and lives with his wife Sarasamma (K P A C Lalitha) daughter Valli and son Selvan. After he retires, Munisamy pays the entire amount he gets as retirement benefits to the Party Secretary as bribe to get his son a job.
However, even after a long wait Selvan does not get the promised job. Disillusioned, Munisamy leaves home one day and does not return. The burden of taking care of Munisamy's family falls on Achuthan. He looks after Munisamy's children as his own.
An honest party worker and an idealist, Achuthan takes up the issue of corruption within the party in party forums. This antagonizes sections within the party and leads to his being ostracized by the party. People opposed to him even hint that he has an illicit relationship with Munisamy's daughter.