Perarasu movie is about CBI officer Kasi Viswanath (Vijayakanth) who is entrusted to investigate the mysterious disappearance of Judge Sadasivam (Nassar). Viswanath has for company two funny assistants (Anand Raj and Pandiaraj).
Kasi finds out that a state minister Elakyam (Prakash Raj) who is behind all crimes in the city and he is assisted by three senior cops. Suddenly one by one the bad cops are killed, with needle of suspicion resting on Kasi as a look-alike of him is behind the murders. At the interval point it is revealed that it is a revenge killing by Perarasu (Vijaykanth) Kasi’s twin brother.