Brian believes the witches are going to sacrifice Jack, a police officer, a la The Wicker Man (1973). This story broadcast in 2006 when The Wicker Man (2006) was remade.
When Rhoda Wishaw shares tea with Brian Lane and Gerry Standing around her camp fire, she sums up the coven's rule of life as "harm none". The full phrase should be "Do what you will so long as it harms none" and it's known as the Wiccan Rede. Here, though, Rhoda confuses it with the more sinister principle of occult Thelema: do what thou wilt shall be the whole of the law. Rhoda's hybrid is "Harm none: that is the whole of the law."