Black Christmas follows a group of sorority sisters who begin receiving increasingly disturbing and obscene phone calls from an unknown caller during their Christmas holiday. One by one, the girls are brutally murdered by an unseen killer hiding within their own house. The film is widely regarded as a pioneering work that helped establish the slasher genre, influencing countless horror films that followed.
Clark's sorority-house killer, the calls coming from inside the house, wrote the slasher template four years before Halloween borrowed it wholesale.