Written by Steve Knight ("Dirty Pretty Things"), the London-set drama begins with a Russian girl who dies in childbirth. A nurse (Watts) investigating her death soon discovers the girl was mixed up in sex trafficking and with the Russian mob.
Mortensen will play a mysterious man linked to the mob.
According to The Hollywood Reporter, Cassel's character, Petrid, comes from a family that owns a Russian brothel.
Cassel's French-language credits include "La Haine," "The Crimson Rivers," "Irreversible" and "Brotherhood of the Wolf." American audiences, though, are more likely to recognize him from films like "Derailed" and "Ocean's Twelve."