Shanmugam (Parthiban) is a veterinary doctor. He is married to Valarmathi (Devayani), a working woman. The couple has two daughters and an adopted son.

The movie drifts back to Shanmugam#$#s school days, when he falls in love with Dhanalakshmi - a girl of the upper caste. She fails in her tenth standard exams, but Shanmugam manages to study well and becomes a veterinary doctor. After his studies, he returns back only to find that the girl he loved had been married against her will to her brother-in-law (Sayaji Shinde) after her sister#$#s death. Shanmugam marries Valarmathy and gets settled.

Shanmugam and Valarmathy#$#s routine life moves along happily. Meanwhile, tragedy strikes Dhanalakshmi and she loses her husband. To support herself and her son, she joins a construction company as a worker.

One day, Shanmugam finds Dhanalakshmi working at the construction site. Unable to bear the sight, he takes her to his house and employs her as a helper to his wife in her daily chores. Slowly, his feelings for his chidhood sweetheart return. His wife starts suspecting his loyalty to her. Dhanalakshmi decides that she should not spoil the couple#$#s healthy marriage, and disappears from the house one night, leaving her son behind.

Thangar Bachan must be appreciated for making a movie filled with emotions. However, the movie also has some commercial touches. Vivek#$#s comedy provides relief in an otherwise serious movie. Ilayaraja#$#s music is good in some songs. The cinematography is pleasant and soothing, a characteristic of all movies made by cinematographers turned to directors.

Azhagi is not a movie meant for entertainment and relaxation. If you appreciate good quality filmmaking and have a taste for art films, then Azhagi is the right movie for you!