[Photo provided to China Daily]

The movie is about the double lives of undercover police agents, who are struggling to return to normalcy, while hunting for moles.

But despite the series receiving a lot of praise, some viewers say parts of the plot don't work.

The shortcomings will be fixed in the movie, says Jazz Boon, the film's Hong Kong director and also the series' executive producer.

"The scale and budget of the movie is much bigger than the series. We have more resources and more time for filming, so the movie will have a much better story," he said last week at a press conference in Beijing.

Work on the movie began in Hong Kong in December, and the crew later traveled to Macao, Shenzhen in South China's Guangdong province and Rio de Janeiro in Brazil.

The filmmakers spent three and a half months on filming.

Cheung says filming in Brazil was the biggest challenge because of high temperatures and security concerns.

"It was my first time to Brazil. Local policemen escorted us to the sets. We could see bullet holes in the walls," he says.