12 B film begins with a young man named Shakthi (Shaam) leaving his house one morning en route a job interview, when he spots a beautiful girl Jothika aka Jo (Jyothika) walking down the street and he's both attracted to and distracted by her. He begins to follow her and in the process, accidentally misses his bus and is mugged. The film then pauses at this point as a voice over begins to tell the viewer that we are about to examine how Shakthi's life will be impacted, by missing the bus or by making to the bus.
The Shakthi who makes the bus, gets to the job interview on time and gets the job of a bank manager and a beautiful young co-worker Priya (Simran Bagga) falls in love with him. The catch there is that this Shakthi is still in love with the same woman who his alternate self (who missed the bus) loves, but try as he may, something keeps preventing him from getting to her. That thing turns out to be his alternate self (that is the him who missed the bus). He is well off in life but miserable because he can't get the girl he loves.
The Shakthi who missed the bus, arrives at the interview too late and is thrown out of the bank. On his way back home, he passes by a junkyard where he spots an old friend (Vivek) who gives him a job as a mechanic and life goes on. The next day, while crossing the street, he spots the dream girl from the day before and begins to follow her again until he is able to get her attention and they begin a relationship. One day, Jo's friend (Sunil Shetty) comes to visit and it turns out he too is in love with her and wants to marry her, but the girl is so much in love with Shakthi and vice versa.
Due to some misconceptions Jo and Shakthi (who missed the bus) separate. On the other end, Priya expresses her love for Shakthi (who made it to the bus). The climax scene is cliched and Shakthi (who makes the bus) while reciprocating his love for Priya is met with an accident. At the same moment Shakthi (who missed the bus) is also grivieously hurt in the same freak accident while he tries to patch up with Jo. Shakthi and his alternate self are admitted in the hospital.
The film ends with Simran wailing over the death of Shakthi (who makes to the bus) catching a glimpse of Shakthi (who missed the bus) patching up with Jo at the hospital.