Aadu Puli Aattam movie is all about Mandhiran (Maniprakash) who is an orphan is forced to become a gangster. He works for a bigwig Sadayan (Damodhara Raju). He, along with his gang, mercilessly kills innocent people, loots them of cash and jewels and kills people for money. A special team of police headed by Vellaithurai (Sudhakar Vasanth) is on their trail.
Mandhiran comes across Gayathri (Vennila) who is a Brahmin girl. After a few encounters, they both fall in love. Mandhiran helps her family win a property dispute and he earns the goodwill of Gayathri's father (Poovilangu Mohan).
The rest is a cat and mouse game between the police and Mandhiran's gang. Whether he marries Gayathri or not forms the climax.