The film features an ensemble cast, including Udhayanidhi Stalin, Hansika Motwani, Aishwarya Rajesh, Radha Ravi, Prakash Raj, Vivek and Krishna Kumar.

The film begins with a drunk Rahul Dewan racing in a Toyota Land Cruiser Prado against an Audi, after he is seduced by a woman in the passenger seat.

Sakthi, desperate to have some money and fame, decides to pursue the case and files a PIL in the court against Rahul Dewan's acquittal.

With the help of his uncle Surya, Sakthi collects the video footage of the Land Cruiser involved in the accident and presents it to the court.

Though Adhiseshan counters him that the Land Cruiser was driven by the driver of the Dewan family, Sakthi refutes his statement by submitting the necessary evidence.

Sakthi and journalist Jennifer arrive at Vandavasi, but the police, who captured Muthupandi, kidnap the survivor.

The next day, Sakthi reveals that he brought the survivor, thanks to Muthupandi intentionally showing the police the wrong man, and Sakthi prosecutes Selvam and finds that he has botched up the investigation by declaring Kamalakannan (Raja Rishi) as dead and snatching Kamalakannan's hard-earned money to stop him from killing Kannan.

After a conclusion of arguments and counter-arguments between Adhiseshan and Sakthi, Dhanapal directs the Tamil Nadu Police to suspend Selvam and initiate a criminal inquiry against him and also declares Rahul as guilty of the crime and sentences him to 14 years in jail under Section 304 and under section 307 of the Indian Penal Code.

[2] However, the team were unable to begin production and decided to stall the venture, instead choosing to remake Subhash Kapoor's 2013 Hindi film, Jolly LLB.

[5] The makers initially sought to cast Akshara Gowda in another major role, before finalising Aishwarya Rajesh.

[12][13] The makers initially thought of titling the film Shakti, after the lead character, until cast member Vivek suggested Manithan.

[20][21] Malini Mannath of The New Indian Express wrote, "The courtroom drama with its twists and turns, is engaging and well crafted.

[22] Lata Srinivasan of DNA India wrote, "Manithan works because of the casting and the fact that one lawyer from nowhere decides to fight for justice for the have-nots.

[23] M Suganth of The Times of India called Manithan, a legal drama, "a welcome relief in this age of ghosts and guns" and rated it 3.5 out of 5.