Rocket Raja (Karthi) is a small time thief who is talented enough to steal anything he comes across. In the company of Kaatupoochi (Santhanam), he enjoys life to its extent and even falls for the charm of a beautiful girl (Tamannaah). However, his life changes forever when a small girl comes to Raja calling him dad. As Raja finds out the girl's past, a flashback reveals that she is the daughter of an honest cop Rathanavel Pandian (Karthi), who is a nightmare to baddies. However, Pandian is now is on the run because his enemies are behind him and his daughter. Eventually, the villains find the whereabouts of Pandian and finishes him off. So steps in Rocket Raja (with the help of Pandian's colleagues who are behind the girl reaching him) into his shoes and finishes the unfinished work of Pandian. He also takes care of his daughter. A roller-coaster ride of fun, joy, camaraderie and romance is what 'Siruthai' is eventually.