Thanmatra movie tells the story of Ramesan Nair (Mohanlal) who is a Kerala government secretariat employee, cocooned in his own small and happy world. An honest and sincere man, Ramesan's family consists of his loving wife Lekha (Meera Vasudev), son Manu (Arjun Lal) who is a plus-two student, and daughter Manju (Baby Niranjana), a primary school student.
His biggest ambition is to see that his son gets into the IAS(India Administartive Service), something he himself had failed to achieve despite being a brilliant student. Manu is a very loving son and an intelligent student who shares a strong emotional bond with his father.
Ramesan starts to develop problems with his memory. What starts as commonplace omissions and absentmindedness, quickly grows into handicapping cognitive and behavioral impairments.In the hospital, Ramesan is diagnosed with Alzheimer's disease, a disease which causes a gradual loss of memory and cognitive abilities.
The news comes as a grave shock for the happy family and turns their world upside down. The family is devastated by the sad news, but tries to adjust to the situation with a lot of determination underscored by strong emotional bonds. How they cope up with the trauma, insecurity and uncertainty caused by Ramesan's plight, forms the gist of the movie.