With a last-minute cancellation by the babysitter, dewy teen Libby Mannering (ANDI GARRETT) is put in charge of her bratty little sister Tess (SHARYL LOCKE) while her parents attend to some sort of business-related dinner out of town. Thankfully, Libby is allowed to invite her best gal pal Kit (SARAH LANE) over to help her pass the unchaperoned time. Rather than sneak alcohol from the parents liquor cabinet, participate in a no-holds-barred pillow fight or invite boys over to listen to records with the bedroom door open, the sensible girls decide to let loose by making prank phone calls. Whoa! In their breathiest of voices the girls repeatedly annoy unsuspecting telephone owners by repeating the phrase, "I saw what you did; I know who you are." Most folks laugh off the girls' inane calls, save for one Steve Marak (JOHN IRELAND) who just finished showering and stabbing his wife to death. The girls', intrigued by Steve's brooding baritone and utterly oblivious to the murder, get all dolled up and decide to drive by his house to get a better look at him. While peering through his window, Libby is chased back to her car by Steve's over dressed and over-the-hill mistress Amy (JOAN CRAWFORD) who thinks the much younger girl is trying to get her mitts on her man. Amy snags Libby's vehicle registration card as a trophy and confronts Steve with it, who, in turn, gets all stabby on her and her over-sized necklace. Armed with Libby's home address, Steve drives over to the Mannering manor to terrorize the girls. Directed by WILLIAM CASTLE, the master of b-movie promotional gimmick, I SAW WHAT YOU DID barely stands up as a cautionary tale against the dangers of trick telephony since the advent of modern caller-id makes the major plot point obsolete. IRELAND brings a sense of brooding menace to his performance, and the young girls all deliver their screams on cue. 

• Taking a page from HITCHCOCK's PSYCHO play book, CASTLE lets his victim intrude on a shower with a killer already in it
• The cougar shows her claws: Amy's front yard confrontation of Libby
• The ominous fog that envelops the Mannering home in the finale
• Early use of the hider-in-the-hatch phenomenon (Remember kids: Always check the backseat of your car for the killer!)