《摩斯探长第四季》讲述的是：Morse finds himself the subject of a murder investigation when his friend, Beryl Newsome, is murdered at a rehearsal of the Magic Flute and he foolishly touches the murder weapon. Morse is suspended and DCI Bottomley, with DS Lewis assisting him, is put in charge of the case. Morse feels that he was set-up and looks to his past to see who, among the many criminals he arrested, might now be setting about seeking revenge. When someone scratches masonic symbols all over his car and he is reported for erratic driving, Morse wonders if Masons may somehow be involved. When a large number of his personal items are found in Beryl's apartment, Morse is placed under arrest.