Pariyerum Perumal narrates the story of a youth hailing from Puliyankulam village near Tirunelveli. Pariyerum Perumal alias Pariyan belongs to an oppressed caste. The movie begins with the brutal murder of his beloved black dog 'Karuppi’ (lit. Blackie in Tamil). The canine hunting companion of Pariyan is the first one we see becoming a victim of caste hegemony.

Pariyan moves on to secure a seat in Government Law College, Tirunelveli where he wishes to become a lawyer like B. R. Ambedkar. He meets Jothi Mahalakshmi alias Jo, a naive girl from Ambasamudram who takes it upon herself to teach English to Pariyan. She eventually falls for his innocence and honesty. This newfound friendship irks Sankaralingam, a fellow law student and Jo's cousin. He awaits a chance to teach a lesson to Pariyan and establish his caste superiority. He does so while the blissfully unaware Jo invites Pariyan to a family wedding.

In another part of town, an old man is seen engineering unique and cunning methods to kill innocent people, for example, strangling a young girl then hanging her and drowning a student in the river. At the wedding that he was invited to, Pariyan is almost beaten to death by Sankaralingam and his fellows, and they also urinate on him. It was stopped, and Pariyan was warned by Jo's father, who asked Pariyan to stay out of Jo's life. Jo, unaware of all this, yells at Pariyan the next day for not attending her sister's marriage as she had invited only him in the whole college. Pariyan's answers made Jo cry and have a fight with him.

Pariyan, who becomes upset the next day, drinks alcohol and attends the class, which leads him to the principal's (Poo Ram) room. He is asked to bring his father, so he bribes a person to be his father, and the man does the job successfully. Another day, Pariyan's teacher tries to solve the problem between Pariyan and Jo, but Pariyan points out that Jo gave an invitation-only to him, making it clear that he treats her only as his friend, which infuriates her. The very same day as he sits in Sankaralingam's bench, the latter gets infuriated, pushes Pariyan into the ladies' toilet, and locks the door, which once again leads Pariyan to the principal's room. Pariyan is again asked to bring his father. This time, he brings his real father, who is a folk dancer who dances in drag clothing.

The principal understands Pariyan's innocence and asks him to be bold and continue to fight like this. As Pariyan comes out of the principal's room, he finds his father brutally ragged by Sankaralingam and his fellow mates, and they pull Pariyan's father's dhoti. Pariyan's father runs out of college half-naked, followed by Sankaralingam and his group, and Pariyan runs after them. Pariya takes his father to a hospital to get him treatment. Meanwhile, Jo's father realizes that his daughter is madly in love with Pariyan. Fearing humiliation and ostracization from his caste if his daughter elopes with someone from a lower caste, he hires the old man("Thatha Maistry"). It is then revealed that Thatha Maistry is an assassin who murders people to protect the honour of the people of his caste. Thatha reveals that he looks at his work as a divine ordinance and promises that the day that he is unsuccessful, he will kill himself.

Thatha and Pariyan know each other, and Thatha even offers to fix the problem by talking to Pariyan. Pariyan, who is by his father's bedside at the hospital, contemplates taking revenge, but his mother asks him not to do so. She reminds him that humiliation and oppression is something that they have dealt with their whole life and that violence won't fix anything. He is informed that Jo is waiting in the hospital for him. An emotional Jo makes Pariyan realize how important he is for her, and though she loves him, she asks him to be the same old Pariyan, who was very friendly with her. Pariyan makes a promise to be like that.

The old man asks for a lift in Pariyan's cycle and tries to kill him. The old man then puts Pariyan on a railway track to kill him in the same way that Karuppi was killed, but Pariyan wakes up and escapes, which leads to a heavy fight between the old man and Pariyan. Pariyan realizes that he was sent by Jo's father and Sankaralingam. He beats Sankaralingam and others brutally. He then says to Jo's father that the life he is living is a gift given by Pariyan because if Jo becomes aware of the activities of her father, she will not spare him. He asks him to leave him and goes. The old man, who realizes that he has failed in his attempt to kill his mark for the first time, commits suicide on the railway track.

Jo, who remains completely oblivious to the violence around her, brings her father to meet Pariyan, and as she goes to buy tea for them, Jo's father asks for an apology for his mistake. He speaks to Pariyan well and then says, "We can see. We do not know what will happen in the future," to which Pariyan replies, "As long as you remain the same and expect me to remain a dog, nothing will change".