Billy (Marina Zudina), an FX make up artist who does not have the physical ability to speak, is in Moscow working on a low budget slasher film directed by her sister's boyfriend Andy (Evan Richards). On one particular night Billy returns to the set to fetch a piece of equipment for the next day's shoot when she is accidentally locked in the studio. Being unable to speak but having the ability to communicate with her sister Karen (Fay Ripley), Billy makes several telephone calls but is interrupted when she discovers a small film crew working after hours to shoot a cheap porno film. Watching unseen, Billy is amused until the performed sex becomes sadistic. When a masked actor pulls out a knife and stabs the actress (Olga Tolstetskaya), Billy reacts and is discovered. She flees pursued by the homicidal film crew.
Billy narrowly escapes and manages to tell her story to her sister and the police, but the snuff film crew manages to convince the authorities that the onscreen 'murder' was a cinematic special effect. However, the events bring forward Larsen (Oleg Yankovsky), an undercover detective who is tracking the activities of the covert film crew and their connection to a shadowy criminal mastermind called "The Reaper". The Reaper (Alec Guinness) is a financier of an international underground snuff ring. He tells the criminal film crew that Billy is a witness and must be eliminated, thus motivating the snuff film director, his thug assistant and a host of subsidiary criminals to retrieve a missing computer disc from her and dispatch it. As more and more factions get involved in killing and saving Billy the action becomes wild and fantastic and hard to discern what's real from movie magic.

Question: What does the masked man do that cases Billy to react?
Answer: He stabs the woman.
Alternative answer: He stabs the actress.