Kali is a winch operator at a village power plant. A reputed troublemaker, he occasionally does good deeds for the local community. Kali and his younger sister, Valli (to whom he is devoted), were orphaned in childhood and have no close family. When a poor wanderer, Manga, and her aged mother arrive in the village with no assets and no one to assist them, Valli helps them set up a home in the village. Although Manga becomes fond of Kali, he is repelled by her fondness for food.

The power plant's new supervising engineer is Kumaran, an austere-but-fair boss. His relationship with Kali is difficult, worsening after he sees Kali's negative side in a series of incidents (including allowing people to ride the winch, in violation of power-plant rules). Kali calls Kumaran "Law Point" because of his strict application of the rules. One day, when Kali is on duty, Manga teases him; he abandons the winch and chases her. In his absence, an emergency arises at the plant. An enraged Kumaran suspends Kali from his job the following day for negligence of duty, ignoring his protests and threats.

Kali gets drunk and passes out in the road; a lorry runs over his left arm, which is later amputated. He cannot do his job with one arm, and is fired. Unemployed, Kali directs his anger and frustration at Kumaran and Manga feels guilty because she is responsible for Kali's plight. At Valli's request, she marries Kali and takes care of him.

Kumaran is attracted to Valli, and asks Kali for permission to marry her. Because he hates his former boss, Kali arranges for Valli to marry Murugesa, a philandering grocer. Although Manga opposes Kali's decision, he stubbornly refuses her plea to let his sister marry Kumaran; she decides to arrange the marriage without his consent.

On the day Murugesa is supposed to marry Valli at his house, Kali learns that the entire village has left to attend Valli's marriage to Kumaran. He intercepts Valli's group, reminding her that he loves her and how difficult it would be to live without her. Although everyone else ignores him, Valli runs to him and assures him that she would never abandon him. Relieved that Valli still respects him, Kali then tells Kumaran that although he still dislikes his former boss, he and Valli have his permission to marry.

• Rajinikanth as Kali
• Sarath Babu as Kumaran
• Fatafat Jayalaxmi as Manga
• Shoba as Valli
• Venniradai Moorthy as Murugesa

Development

Mullum Malarum is based on a novel of the same name written by Umachandran, which was published in the Tamil magazine, Kalki. The story, about the love between a brother and sister, won first prize in Kalki's Novel Short Story Competition for the magazine's silver jubilee in 1966. Screenplay and dialogue writer J. Mahendran read only part of Umachandran's novel, but was particularly impressed by the winch operator Kali's affection for his sister and the loss of his arm. He outlined Mullum Malarum to producer Venu Chettiar, who was also impressed. Adapting Umachandran's novel for the screen, Mahendran made his directorial debut and wrote the film's screenplay. The screenplay diverged from the novel's plot, and Mahendran decided to make a minimalist, visually-rich film without melodrama, overacting, excessive dialogue or duets. A significant difference between the novel and the film is that in the novel, Kali loses his arm to a tiger; in the film, he loses it when he is run over by a lorry.

Chettiar and V. Mohan produced Mullum Malarum for Ananthi Films, although only Mohan appeared in the opening credits. Ramasamy was signed as art director, and D. Vasu as editor. Ashok Kumar and Ramachandra Babu were the initial choices for cinematographer, but actor Kamal Haasan recommended Balu Mahendra for the job; Mahendra accepted, making his debut in Tamil cinema.

Casting

Chettiar disagreed with Mahendran's desire to cast Rajinikanth in the lead role because of the actor's dark skin and typecasting as a villain. Mahendran refused to direct the film if Rajinikanth was not cast—not because Rajinikanth was a close friend, but because he felt that the actor was perfect as the character; Chettiar reluctantly agreed. He was still unhappy with the director's decision, and called it "ridiculous" and "preposterous" every time he visited the set. Rajinikanth, dismayed at Chettiar's lack of confidence in his acting, promised to "put his heart and soul into the character Kali".

Shoba was cast as Kali's sister Valli, Sarath Babu as the engineer Kumaran and Fatafat Jayalaxmi as Kali's love interest, Manga. Inspired by Sringeri's marine environment, Mahendran characterised Manga as a "foodie who loves fish". Latha said that she had to refuse a part in the film due to scheduling conflicts. Mahendran cast Venniradai Moorthy as Murugesa, the philandering grocer, and Samikannu was given a role as one of Kali's henchmen. Male supporting roles were played by S. A. Kannan, Pazhaniyappan, Dasaradan, Rangamani, Sarathi, Santhanam, Kumarimuthu, Jothi Shanmugham, Chellappa, Amalraj, Poondigiri and Vairam Krishnamoorthy. Female supporting roles were played by Santhamma, Jayakumari, Vijaya, Jaya, Pushpa, Radha, Prema, Vasanthi, Leela and Kala.

Filming

Mullum Malarum was filmed on 35 mm ORWO colour film. The film defied traditional Tamil cinema conventions, discarding elements which Mahendran disliked. Mahendra later said he avoided incorporating the usual hero-heroine dancing into the film because he thought it was like "watching two drunken monkeys dancing". Instead, he allocated music to the background when the lead characters expressed their emotions. Since Mahendran had no previous directing experience, Mahendra assumed responsibility and responded to Mahendran's suggestions for screenplay, dialogue, camera angles, casting and editing.

Mullum Malarum was shot primarily in Sringeri, with additional filming in Ooty; filming lasted about 30 days. Chettiar initially refused to let Mahendran shoot in Sringeri (citing financial constraints), but Mahendran's friend Pazhaniappan convinced Chettiar and agreed to pay for the Sringeri shooting schedule. Chettiar held up production by not financing a scene set before the song "Senthazham Poovil", but Haasan funded the scene. Sarath Babu was originally supposed to lip sync the full "Senthazham Poovil" scene with him and Shoba, but Balu Mahendra and Mahendran agreed on a montage after the actor performed a line or two; Babu was disappointed. Chettiar was puzzled by the finished film's lack of dialogue, since he hired Mahendran as director due to his success as a screenplay and dialogue writer and did not expect such a visually-rich film. Mullum Malarum's final length was 3,915.46 metres (12,846.0 ft).

Film critic Naman Ramachandran notes that Umachandran's novel and Mahendran's film metaphorically liken the sibling relationship to flowers, which need thorns to protect them. According to film critic Baradwaj Rangan, "Mullum Malarum" can be read in two ways: "as a descriptor of the siblings (in which case, it translates as the thorn and the flower), or as a pointer to the end, where the brother softens (even a thorn will bloom)". S. Rajanayagam wrote in the 2015 book Popular Cinema and Politics in South India: The Films of MGR and Rajinikanth that Kali is a thorn and a flower throughout the film: an "angry young man with a kind heart" who does not admit mistakes (despite many, such as breaking car headlights and allowing people to ride the winch, in violation of the power plant's rules). He noted that films like Mullum Malarum stereotype the poor as representing "all that is pristine and traditional": "The overall socio-economic system, which has made them poor, is unchallenged. Within the system, however, the hero will be 'richer' in terms of his moral uprightness." The 2012 book Grand Brand Rajini by P. C. Balasubramanian and Ram N. Ramakrishnan describes Kali as "the loving brother, the angry worker and despondent physically challenged person rolled into one." The Encyclopedia of Indian Cinema by Ashish Rajadhyaksha and Paul Willemen compared the film to Pasamalar (1961) for providing "a low-key version of the orphaned brother and sister theme".

According to French film historian Yves Thoraval, the film's theme is "the Oedipal possessiveness of a married brother for his younger sister". Ramachandran notes that, like the characters in Bairavi (1978), Kali and his sister have abusive parents and he is responsible for his sister's welfare. Unlike Bairavi, the siblings in Mullum Malarum are not separated; this leads Kali's protectiveness of Valli to the brink of obsession. In one scene, after he violently berates her during the day he puts henna on her feet at night while she sleeps.

Ramachandran regards egotism as one of Mullum Malarum's central themes, with Kali (the community's alpha male) surrounded by sycophants who compliment him. He plays God, allowing the villagers to ride the winch (saving them the exertion of walking), and is jolted when supervising engineer Kumaran arrives. As a subordinate, Kali cannot oppose Kumaran; his frustration threatens to erupt several