McKellen and Condon previously worked together on the drama "Gods and Monsters."
Based on the novel by Mitch Cullin, the screenplay for "Slight Trick" is being adapted by Jeffrey Hatcher. It tells the story of how the aging Sherlock tries to solve the decades-old mystery that has haunted him for years, The Hollywood Reporter said Thursday.
The film is to start shooting in April in the United Kingdom.
The project was announced this week at the Toronto International Film Festival.