There's an art to quality schlock and "Knock Knock," the second new film from Eli Roth in the past month after "The Green Inferno," offers plenty of it.

The film is tightly constructed, with a classic B-movie premise that unfolds largely within a single setting, narrowing the filmmaker's focus and letting him show off his skills without taking the "Hostel" approach of overdosing on gore.

It concerns a wealthy architect (Keanu Reeves, coming off the best B-movie in years, "John Wick"), married and a father, who finds his will tested when a "real-life" porn scenario presents itself: two beautiful women (Lorenza Izzo, Roth's wife, and Ana de Armas) show up at his door one rainy night and seem weirdly, aggressively interested in a joint physical pursuit, to put it mildly.

There's simply not enough here to sustain a feature. Roth, who co-wrote the picture with Guillermo Amoedo and Nicolás López, builds a sense of eroticized mystery in the first third that devolves into mundane exploitation as the reality of the situation comes into focus.

Roth takes great pleasure in tormenting Reeves' protagonist from beginning to end and the movie indulges in taboo instincts to the greatest possible extent for a mainstream production.

There's undeniable schadenfreude as the camera lingers on the destruction of the character's nouveau riche mansion and all its accoutrements.

But the characters aren't really characters -- they're archetypes -- and the movie plays as a softcore lark rather than more substantial entertainment. It's a horror twist on a porn premise that flatlines once the former element takes over.