The story is about three different women who work at a fashion export company. Janaki (Urvashi) is a typist who lives with her husband and her baby to earn for a living as her husband lost his job in a factory. Pappamma (Rohini) is a house keeper who lives with her drunkard rickshaw pulling husband and earns for both their living and her husband's drinking expense. Satya (Revathi) a computer graduate and designer who waits to get married. Her marriage was stopped by her once when the groom's family listed a lot for dowry. She joins the fashion company as a fashion designer and stays in a hostel. All the women in the company face a common threat in the form of manager Pandian (Nassar) who is a womanizer. He harasses his female employees in one way or other but the women tolerate his torture due to their family circumstances.

Pandian tries to move close with Satya and unaware of Pandian's intention Satya at first dines with him. This isolates her from other employees. Pandian tries to impress her by presenting a silk saree but Satya insults him by saying that only a father or husband should present saree to a woman and what he has done is pure idiotic. This gains the friendship of Janaki and Pappamma to her and Satya understands about Pandian. A small daily clash occurs between Pappamma and Janaki whenever Pappamma misplaces rat poison besides sugar and Janaki mixes rat poison by mistake in the coffee of Pandian's as both sugar and poison look alike. Due to Satya's friendship Janaki becomes bold enough to face her supervisor and she insults her in front of everyone. Hence the supervisor tries to revenge by hiding an important file which is under Janaki's custody. But Pappamma helps her by revealing the truth to Pandian. All the three women are much frustrated by Pandian's attitude and are finding some way to stop his misdeeds towards them.

Pandian one day tries to hold Janaki after office hours. But Satya stays back for her help. Janaki mixes rat poison without seeing the stuff in Pandian's coffee and gives him and goes for her work. Pandian falls from his chair and becomes unconscious. Satya finds his state and admits him in hospital. Janaki finds that she has mixed rat poison in the coffee and Pandian drank it and reveals the truth to her friends which shock them. All three rush to hospital and hear that the patient is dead due to strong dosage of poison. But actually they don't know that the dead man is a terrorist who died by drinking cyanide and Pandian has actually fainted due to minor injury and has recovered. They kidnap the corpse and take it to Satya's room only to find that they have kidnapped corpse of the terrorist. Hence they decide to place it back in the hospital. But Pappamma's husband misunderstands that his wife and the dead man has a relationship and fights with him. Finally police find out the missed corpse and all three leave silently. Pandian returns to office and this shocks the three women. The supervisor overhears their conversation and informs Pandian.

Pandian blackmails them that he would report to the police about the murder attempt if they did not obey what he says. He demands to spend each night with all the three women in his private house. All three come to a decision and pretend to accept his demands. In his private house, they initially act to romance but finally trap him and tie him. Satya takes charge of acting manager and with the help of Janaki, Pappamma and other employees she brings many changes in the office. In the evening all the three go the private house. One day Satya receives information from head office about boss' visit and get tensed as they have kidnapped the manager. All the three decide to confess about whatever happened in the office and rush to airport to receive the boss. The supervisor overhears them and she rescues Pandian. Both Satya and Pandian try to rush airport before the other. Satya mistakes for an old man to be the boss instead a young man (Kamal Hassan) comes and leaves with Pandian. The trio is shocked and chase him to office.

In the office the young boss finds differences and Pandian says that he is not responsible for anything happened here. But the boss looks like he is aware about whatever happened there and appreciates all the three for their innovative ideas. He decides to leave the charge of the office to the trio and transfers Pandian to Andaman. He also gives remedy for Janaki's husband's job in his friend's factory and Pappamma's husband's job as watchman in their school. When he inquires about Satya's marriage Janaki tells that she has a dream boy drawn in the computer. Satya shows her dream boy to him and all are surprised that Satya's dream boy is their boss. The boss asks Satya to marry him and they both fall in the repaired chair revealing that Satya accepted his proposal.

• Revathi as Sathya
• Urvashi as Janaki
• Rohini as Pappamma
• Nassar as Pandian
• Renuka as Pandian's wife
• Crazy Mohan as First Doctor at Hospital
• Madan Bob as Doctor
• Kamal Hassan as Boss from Head Office (Cameo appearance)
• Nagesh as Deadbody
• Pasi Sathya as Madhavi
• R. S. Shivaji as Piles patient
• Thalaivasal Vijay as Papamma's Husband
• C. R. Parthiban as Police inspector
• S.Thanu as Supervisor Thamizhavan
• Santhana Bharathi as Father of the groom

Rohini was offered to play the role of de-glamorised servant; she recalled that she agreed to do "without a second thought." Producer S. Thanu appeared in a small role as office supervisor, notably it was his only film as an actor. Tirru was recruited as cinematographer. Velraj worked as an assistant cameraman in the film. Kamal Haasan who scripted and produced the film appeared in a small role as office boss towards the climax. Nagesh did a small role as corpse.^[6]

Magalir Mattum was dubbed in Telugu as Aadavaallaku Maatram and Ladies Only in Malayalam. The film was remade in Hindi as Ladies Only in 1997 by Dinesh Shailendra. Seema Biswas, Shilpa Shirodkar and Heera Rajagopal were cast as three working women and Randhir Kapoor was selected to reprise Nassar's character. Kamal Haasan portrayed Nagesh's dead body character. Even though the film has been completed in 1987 it was already released in 1994.^[7][8]

Soundtrack is composed by Ilaiyaraaja while the lyrics for all the songs were written by Vaali.^[9]

• Karavai Maadu — SPB, S. Janaki
• Veetai Thaandi — S. Janaki
• Magalir Mattum — S. Janaki
• Mottu Mottunu — S. Janaki