Prabhu, an orphan, is brought up by his friend Murali's mother and is the general manager for Murali's marketing agency at Chennai. Murali treats Prabhu not just as a friend, but as a brother and depends on him for all business decisions. Prabhu is in love with Padma, a college student. Padma's relative Jayanthi is a young widow who works at Murali's office. Murali, an alcoholic, with a roving eye is not aware of Prabhu's love for Padma. Padma is the only girl who rejects Murali's advances. Though Jayanthi knows about the love affair between Prabhu and Padma, Prabhu's presence stimulates her sexually. Once she accompanies Prabhu and Padma for a film screening and unable to control her sexual feelings, comes out. Murali offers her a lift in his car; both get excited in the privacy of the car, but suddenly feel guilty about the incident.

A disturbed Jayanthi avails leave and goes to her village and Padma joins her, as Prabhu goes on an official tour. Prabhu, planning to surprise Padma, proceeds to Jayanthi's village to meet Padma. When Prabhu lands at Jayanthi's home, she is alone as Padma has gone to attend a wedding in a nearby village. Jayanthi convinces Prabhu to stay back overnight, so that he can meet Padma the next day. Jayanthi arouses Prabhu in the night and they make love. Prabhu, feeling guilty about the whole event, writes a letter of apology to Jayanthi and leaves. Devastated to read the note when she returns to Jayanthi's house, Padma returns to Chennai, where she starts distancing herself from Prabhu without revealing why.

Murali continues to pursue Padma. When she falls down, he saves her and gets her admitted in a hospital, impressing her father. Murali happily informs Prabhu about his feelings for Padma; being a good friend and grateful for the help given by Murali's family, Prabhu remains silent. He accompanies Murali's mother to formally propose Murali's marriage with Padma. Padma's father accepts the proposal, and Padma remains silent. When Prabhu confronts her, she reveals that she had read his letter to Jayanthi, who is carrying his child. Shocked with the news, Prabhu leaves Chennai without intimating Murali and his mother and they feel betrayed. Prabhu searches for Jayanthi and finally traces her in Bangalore.

Prabhu makes arrangements to marry her, but she meets with an accident. Just before she dies, he ties a thaali around her neck and gives her the dignity of a wife. He goes to Ooty and works in an estate. Meanwhile, Murali and Padma's wedding is postponed, as Murali falls sick and is advised rest by his doctor. He and Padma come to the same estate on holiday, not knowing that Prabhu is working there. Prabhu meets them and apologises to Murali, who is in no mood to forgive him. But Padma, on learning that Prabhu married Jayanthi before she died, forgives him and tries to come close to him again. Seeing them together, an enraged Murali takes Prabhu to a cliff; when he is about to shoot Prabhu, Padma reveals that they were originally in love and how they broke off. Murali presents Padma as a gift to Prabhu and leaves to Chennai. Prabhu and Padma finally unite.

• Kamal Haasan as Prabhu
• Rajinikanth as Murali
• Sripriya as Padma
• Jayachitra as Jayanthi

Ilamai Oonjal Aadukirathu was produced under the banner Sri Chitra and directed by C. V. Sridhar, who also wrote its story and screenplay. It was also the first collaboration between him and Rajinikanth. Editing was handled by Kottagiri Gopalrao.

The music was composed by Ilaiyaraaja and the lyrics were written by Vaali. The song "Ore Naal Unnai Naan" is set in the carnatic raga known as Pahadi, and "Kinnaththil Then" is set in Vakulabharanam. Jazz trumpeter Frank Dubier and his friend Stephen Lazarus (a gospel trumpeter) played the instruments in the song "Ennadi Meenakshi". The song "Thanni Karuthirichu" was remixed by Achu Rajamani for Ennai Theriyuma (2009).^[9][10] A remix version of "Ennadi Meenakshi" is featured on music artist M. Rafi's album Aasaiyae Alaipolae.^[11] In May 2015, music director Bharathwaj commented on "Ennadi Meenakshi", "I’ve always believed that songs should express some emotion, some feeling. This song is a perfect example of what I believe in. It’s simple, yet so dramatic. The element of drama in this song is so inherent to this film."^[12]

Ilamai Oonjal Aadukirathu was released on 9 June 1978.^[14] Ananda Vikatan, in a review dated 25 June 1978, rated the film 57.5 out of 100, praising Sridhar's screenplay and the limited usage of dialogues. The review wrote that Sridhar proved that Tamil films could be made without a comedy subplot and a separate villain.^[15] Following Sridhar's death in 2008, Sify said, "Who can forget the soft romantic Kamal- Rajini film Illamai Oonjal Aadirathu (1978)?".^[16] The ran for 175 days in theatres,^[17] and won the Tamil Nadu State Film Award for Best Film.^[18]

Sridhar later remade the film in Telugu as Vayasu Pilichindi, which released two months after the original, and in Hindi as Dil-e-Nadaan (1982). While the same cast returned for the Telugu version, the Hindi version starred Rajesh Khanna, Shatrughan Sinha, Jaya Prada and Smita Patil.^[3]