S4·E5 Deep Waters

A sub-postmistress is fatally stabbed in a small-scale robbery, and Frost wastes a lot of time questioning the leading suspects. Then, Helen Tudor, a beautiful first-year psychology student and keen swimmer at Denton University, is attacked on the campus, and suspicion initially falls on her psychology tutor, Keith Michaelson, but Frost's investigations uncover a link with the death of another beautiful young girl swimmer two years earlier, in another town.