S1·E6 The Depths

Some water Links, nicknamed Mermen, from two years ago have reappeared and are victimizing young women. The merman emits a high pitched sound which temporarily paralyzes the victim thus making it where the merman can empty the victim of their soul. This leaves the victim in a semi-vegetative state. The catch is that it only works on virgins. A tie-in to the story is that Kate's younger sister is visiting and dating a rather mysterious guy. Kate starts to suspect the guy may also be a merman and that her sister may be the next victim.