I Start Counting (1969)
When two teenage girls fantasize about their sexual yearnings, one is stalked by a psychopathic murderer. Wynne (Jenny Agutter) is adopted by a family and soon has a massive crush on her stepbrother George (Bryan Marshall). Her friend Corrine (Clare Sutcliffe) is Wynne's virginal friend who alludes to being sexually experienced. The two go into the woods where Corrine is raped and killed by an unknown assailant who has terrorized the countryside with his brutal carnage. When George is accused of the crime, Wynne must prove he is innocent in this thrilling slasher feature.