Big B begins with the murder of Mary Teacher, who has been a messiah for orphans. Mary John Kurishinkal, lovingly called Mary Teacher and Teacher Amma by many, is murdered on the street at night by some professional murderers. A young boy accidentally shoots this on his handy cam.
Mary Teacher has been looking after lots of orphans, but she had brought up four such orphans as her own adopted sons. They are - Bilal, Eddie, Murugan and Bijo. While Eddie and Bijo live in Kochi itself, Murugan is a stunt-director in films and is always traveling. Bilal, the eldest, had left Kochi a couple of years back after being involved in a stabbing incident, following which Mary Teacher had refused to let him enter the house.
Bilal had landed up in Mumbai and had been working as a bodyguard. Murugan and Bilal rush to Kochi. Bilal is determined that those behind the murder should be traced out. Meanwhile the Police, led by Assistant Commissioner Balaji Shakthivel and assisted by C.I. George, too are probing the case. Bilal uses his own resources and begins his hunt, with assistance from Eddie, Murugan and Bijo. Eddie, who has a wife and children, is often reluctant and does not cooperate fully with the hunt for the murderers. Bilal, Murugan and Bijo however just move on, cracking professional criminals, using their links in the underworld, especially since Bilal had been a dreaded name earlier in the city. And their hunt takes them to some surprising revelations.