Simplify the following text for a Grade 12 native English-speaking person.
Original: Based on Serge Brussolo 's bestselling French novel `` Les Emmures , '' about a demolition company rep Sam ( Mischa Barton ) who supervises the razing of a mysterious building and discovers horrifying secrets and past inhabitants entombed within its walls by a vicious murderer .
Simplified: The plot of the movie is based on the best-selling novel `` Les Emmures '' and is about a woman ( Mischa Barton ) who discovers past residents who were entombed within the walls of a building by a vicious murderer .