The film begins with a young man named Shakthi (Shaam) leaving his house one morning for a job interview, when he spots a beautiful girl Jothika aka Jo (Jyothika) walking down the street. 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 catching the bus.
Shakthi who makes the bus, gets to the job interview on time and gets the job of a bank manager and a beautiful young colleague Priya (Simran) 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 late at the interview and is thrown out. On his way home, he passes by a junkyard where he spots an old friend Madhan (Vivek) who gives him a job as a mechanic and life goes on. The next day, while crossing the street, he spots the 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, Aravind (Sunil Shetty) comes to visit and it is revealed that he too is in love with her and wants to marry her.
Due to some miscommunication, 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 meets with an accident. At the same moment Shakthi (who missed the bus) is also grievously 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 Priya 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.Shaam as ShakthiSimran as PriyaJyothika as Jothika 'Jo'Vivek as MadhanMoon Moon Sen as Jo's mumSunil Shetty as Aravind (Voice dubbed by Suriya)Srinath as BoltMayilsamy as CitizenShiva in an uncredited role
Shaam had modelled for four years in Bengaluru before model coordinator Biju Jayadevan introduced him to director Jeeva, who was auditioning for a debutant for his debut venture, 12B. Shaam recollects that during his first meeting with Jeeva, he handed over his portfolio and introduced himself in English, when Jeeva heard him out, and then said, “Repeat what you just said, in Tamil" and Shaam did, and was subsequently signed on the next day after a meeting with Producer Vikram Singh. The promos of the film were critically praised with Shaam signing on to appear on several projects before 12B even released. The film also brought together two of the Tamil film industry's leading actresses in the period in Simran and Jyothika. The pair had previously co-starred in S J Suryah's Vaali (1999) but had shared no scenes, for which Jeeva had been the cinematographer. Vivek also played a pivotal role of a mechanic while Hindi actors Sunil Shetty and Moon Moon Sen were also signed to appear in supporting roles. The film also featured Jeeva's assistant director, Srinath, in a minor supporting role; while Shiva who later appeared in the lead role in Chennai 600028 and Tamil Padam, makes a cameo appearance in a party scene. Shaam's voice was dubbed by television actor Vishwa, Sunil Shetty's voice was dubbed by Tamil actor Suriya, and Savitha Reddy dubbed for both Jyothika and Simran.
While cinematographer Jeeva made his debut as a director, the Producer Vikram Singh for his banner Film Works also worked on a Tamil film for the first time. 12B featured Harris Jayaraj scoring the music, while Thotta Tharani designs the sets, editing is by Lennin-Vijayan, dance choreography by Raju Sundaram and stunts arranged by Vikram Dharma. Jeeva revealed that the film would be titled 12B after a bus he used to take during his college days. The basic premise of the film was taken from the 1998 English film, Sliding Doors by Peter Howitt, which also follows alternate tracks. A romantic song was shot on Shaam and Simran at picturesque locations in Jordan, and this became the first time that a Tamil film had been shot there, with Rekha Prakash choreographing it. Some sequences needed to be shot in the USA with about 300 dancers involved, buy the team later recreated the sets in a studio in Chennai.
12B opened to above average reviews, with the critic from The Hindu mentioning that the film is like "moving through a maze, because for many it could be confusion confounded, at least for the most part of the first half". About performances, the critic added that "Shyam in the hero's garb is an apt choice and looks more like a Madhavan clone and for a newcomer, Shyam is absolutely at ease in dance and fights", while "Simran does a commendable job in the climax".
Rediff.com concluded "full marks to the intention - considerably less for the execution" and went on to praise the performances and the technical aspect of the film, while mentioning that a "drawback would be the languid pace -- there is not enough tension built into the film and, for large chunks of time, the story remains static, with the result that you do not empathise with the characters" and specially mentioned Simran's portrayal as Simran continues with her policy of shifting gradually from glam roles to the more sedate, serious ones and proves to have what it takes.
The film performed modestly at the box office, with average collections reported. Jeeva later went on record stating that he felt the film had not been promoted properly and could have done better business had it been.
The soundtrack was composed by Harris Jayaraj. This was Harris' second release following the highly successful Minnale. 12B songs were also huge hit and remained chartbusters which elevated Harris' popularity.