Dibakar Banerjee's tryst with Byomkesh Bakshi, created by Sharadindu Bandyopadhyay in 1932, began when he was eight: his stern grandmother had forbidden him from reading the stories of the popular detective because he was too young to be exposed to the dark world of crime. The warning had the opposite effect. Banerjee, a Delhi boy, stole copies from reading clubs and devoured the stories where nobody could catch him. "For me, Byomkesh was an adventure, an evocative discovery of the Calcutta of the '40s and my first tryst with sadhu bhasha (a chaste form of Bangla, it was used in literature till the first few decades of the 20th century; erudite Bengalis would choose to converse in it till recently)," says the producer and director of Bollywood films.

Thirty-seven years after this initiation, Banerjee is giving shape to Byomkesh's adventures on celluloid. His 140-minute Hindi film, Detective Byomkesh Bakshy!, will release sometime in April - it was to be released in February but the distributors wanted the film to be adequately promoted. It has been co-produced by Yash Raj Films along with Banerjee, so there will certainly be a high-octane marketing campaign closer to the release date. For Banerjee, a dropout of the National Institute of Design and a former advertising executive, who has made critically-acclaimed films like Khosla Ka Ghosla (2006), Oye Lucky Lucky Oye (2008) and Love, Sex Aur Dhoka (2010) and delivered a dud in Shanghai (2012), this could be his biggest gamble ever.

under wraps. Banerjee lets out that he has combined two novels in the film but won't say which ones. It is reasonable to speculate that one of them is, the first Byomkesh novel Bandyopadhyay wrote in 1932. In the film, Byomkesh will be pitted "against an evil genius who is out to destroy the world," says the YRF website. "It's his wits against the most villainous arch criminal the world has seen, in a world of murder, international political intrigue and seduction." The film's online poster tells viewers to "expect the unexpected".

The teaser on YouTube is impressive - Detective Byomkesh Bakshy! has the look and feel of a cerebral masala film. "From the teaser you can make out that the execution of the idea and the production values are great," says film maker Anurag Kashyap. To play the detective with razor-sharp intellect, Banerjee chose newcomer Sushant Singh Rajput. A dropout from the prestigious Delhi College of Engineering, Rajput had no idea of the Bengali detective or Calcutta of yore. He spent a couple of weeks with Banerjee in the city, which helped him tune into the Bhadralok life: how to tie a dhoti, sit on a pidi for dinner, eat street food, sip tea from earthen pots, roam around College Street and do adda at the famous Coffee House where generations of Bengalis have discussed threadbare all of mankind's problems - from the French Revolution to Sunny Leone.

It still won't be easy for Rajput and Banerjee. Byomkesh is a thinking man, and there is little violence or action in his stories - a film maker's ultimate nightmare. Satyajit Ray, who made Chiriyakhana (1967) with Uttam Kumar as Byomkesh, found the character too verbose and not cinematic enough. Thereafter, he stuck to Feluda, the detective he had created. Aware that this could limit the appeal of Detective Byomkesh Bakshy!, Banerjee has decided to focus on his early adventures. His Byomkesh, unlike in Bengali movies, will be just 24 years old - fresh out of college. "I am portraying the early Byomkesh, not the older mature detective others have shown. After all, Bandyopadhyay wrote Byomkesh over a period of 37 years," he says.

Banerjee's interpretation of the character too is different: he is a detective alright but also a human being with vulnerabilities. This was indicated by Bandyopadhyay in various novels. Thus, Byomkesh falls in love, suffers a nervous breakdown and then recovers from it. "One can just conjecture why an orphan who learnt Sanskrit and Bangla and who could have been a teacher choose to become a detective. Was it a sense of righteousness or did it stem from some insecurity?" says Banerjee who feels these questions make Byomkesh real and human. This gels with Bandyopadhyay's insistence that Byomkesh wasn't a detective - he was a satyanveshi, or seeker of truth. A detective is not always a seeker of truth, but detects what others do not. A satyanveshi unravels mysteries. Some Byomkesh stories, therefore, do not end with the wrongdoer getting arrested or convicted. Truth, after all, has many perspectives and is more mysterious than mere facts.

The film is set in 1943 when the Japanese bombed Calcutta. The challenge for Banerjee was to get the sights and sounds of that period right. To begin with, he had to find the boarding house where the young Byomkesh meets his associate and friend for life, Ajit, for the first time. Bandyopadhyay had located it in his novel somewhere in the outskirts of central Calcutta, adjacent to China Town. Banerjee took months to eventually discover the address as 66, Harrison Street. He realised that it was the boarding lodge where Bandyopadhyay had stayed in 1919-20 when he shifted from Munger to Calcutta to write his matriculation exams.

Banerjee did not get permission to shoot in the building that is over 100 years old. So, he recreated it and the streets around it at Nalasopara on the outskirts of Mumbai. The earlier plan was to shoot the movie entirely in Kolkata, but that was proving to be very expensive. Also, as he had to shoot in crowded streets, it would mean getting permission only on weekends which would have stretched the schedule for nearly four months (it was eventually shot in 60 days flat earlier in the year). So, different areas of the city were recreated at Nalasopara, including the original China Town in Tiretti Bazaar, complete with its famous Nanking Restaurant.

To recreate the Calcutta of 1943, Banerjee and his team scoured through over 1,000 photographs shot by American GIs during World War II of the city which gave them some idea of the dresses, the streets and the buildings. The other source was movies. "I saw movies of the '30s like Mukti, Doctor and Jamai Babu to understand life in those times," says Banerjee. Research took them to unusual places like the record offices of Kolkata Police's detective department. Historian Tapan Raychaudhuri's Bangalnama came in handy, so did the invaluable Street Cries of Calcutta. French documentary maker Louis Malle's film on Calcutta provided some more clues on life in the '40s.

There were many interesting takeaways from this research. One, Banerjee says he realised that in those days there were very few women on the streets - the movie had to reflect that. Two, Kolkata streets were cleaned with water twice a day, at 5 am and 4 pm; so the streets had to be watery for the evening and morning scenes. Three, many of the old cries like the fakir's "Mushkil asaan" or the bard singing Shyama sangeet that had disappeared from Kolkata were resuscitated to give an authentic feel to the film. Banerjee also built a road with pebbled walkways to recreate the roads of the '40s.

Even at that time, many buildings in Calcutta were over a century old. Banerjee's team used innovative ways to show ageing buildings: for instance, flour was pasted on the walls and then treated with heat to get flakes. Drains were filled with sludge to look real. Dust was blown on the sets by using storm fans, water cannons were activated and smoke from small fires was blown in. The VFX machine in the studio mixed these pictures to give the feel of diffusion in the air. It also gave it a certain mysterious look to the scenes.

Banerjee says the Chor Bazaars in Kolkata and Mumbai yielded amazing treasures for the film: old Edison bulbs which were used to light up the streets in the set, Parker fountain pens, old glasses, wrist watches which people kept in their pockets and even a gripe water bottle. "I spent nearly half a day trying to find the right fountain pen which could be used at the Chor Bazaar in Mumbai," says Banerjee. To make the street scenes look authentic, Banerjee hired over 400 junior artists, dressed them in period costumes, and made them jostle for space on the streets with 40 vintage vehicles of that era. Four barbers were on call to give that '40s haircut.

Also, the transportation system had to look like the '40s. Banerjee got help from the Kolkata Police who loaned him a bike - with a sidecar - kept in their museum that could still run. Two tramcars of Kolkata Tramways were picked up from the junkyard and restored to look like what they did in their time. The defunct tracks in BBD Bagh in the centre of the city were revived and the two cars where plied on them. Experts in Mumbai worked overtime for 45 days to build a bus from scratch which could be converted from a double decker to a single decker in two hours flat. Advertisements from the 40s were glued on its body. "For characters like Byomkesh, which are etched in our minds from the books that we have read, directors need to exercise extra caution so that there is not a huge difference between the reel Byomkesh and the book," says film maker Anjan Dutt whose Bangla film, Byomkesh Phire Elo, the final part of a trilogy, hit the screens yesterday.

Banerjee had to take some cinematic licence. For instance, the movie is set in the winter of 1943, though the Japanese bombs rained on the city in 1942. The original story plan was to set the movie in summer, but Banerjee chose winter because people mostly wore white dhotis and shirts in summer to ward off the heat, which did not make great cinematographic appeal. Also it was consciously decided that the movie would not stretch to the whole of 1943 as West Bengal was struck by a famine in the second half of that year. Banerjee did not think it would appeal to the young audiences to depict a detective working in the midst of a famine - the two would be incongruent.

Banerjee is now finalising the film's trailer. The soft-spoken director whose office is in a heritage building in Mumbai's Lalbaug area (it's owned by a Marwari from Kolkata who rents it out only to "creative people") confesses that this is the biggest movie of his career. If it flops, Byomkesh's popularity will remain confined to Bengal. Will it sell outside Bengal? A Hindi serial made by Basu Chatterjee and with Rajit Kapur as the detective was a big hit on Doordarshan over 20 years ago. That augurs well for the film.

Meanwhile, Byomkesh has changed Banerjee. Earlier, it was not unusual to find the film maker in grubby trousers and shoes. Not any longer. His partners in YRF have advised him to be in smart casuals became Detective Byomkesh Bakshy! is out-and-out a commercial film.

Arindam Majumder and Urvi Malvania contributed to this report