Black Christmas is a suspenseful horror film that follows the events at the Pi Kappa Sigma sorority house during their Christmas party. The house mother, Mrs. Mac, receives disturbing obscene phone calls from a mysterious caller known as "Billy."
Some of the girls, like Jess Bradford, are unsettled by the calls, while others, like Barb Coard, find them amusing. Phyl Carlson doesn't think much of them, and Clare Harrison is offended. Jess, who is pregnant and keeping it a secret from her boyfriend Peter Smythe, is particularly troubled by the calls.
After Barb provokes Billy with a snide comment, one of the girls goes missing the next morning. The police initially dismiss it as a missing person case, but when more incidents occur at the sorority house, they start to investigate.
Lieutenant Kenneth Fuller and his team place wire taps on the sorority house phones, uncovering more disturbing information. The calls become increasingly personal, leading to suspicions that the caller may have a connection to one of the girls. The mystery deepens as the caller reveals intimate details that only those involved would know.