It's a shame that there are a few minor plot problems with "The Marathon Murders" as its "Play Misty for Me" sort of plot is pretty exciting. Still, it is pretty interesting and neat to see Barnaby acting like an action hero at the end!
The story begins at a weekend marathon encounter group. Apparently, this therapy group runs the entire weekend, but unexpectedly Lorraine leaves...feigning illness. However, what she really is going to do is kill a group member's wife. After all, she's fallen in love with him and he did say he hates the wife!
After killing the wife, Lorraine essentially tells Ken that she killed his wife....and Ken is apparently too scared of her to tell the cops! And, she threatens to accuse him of the murder if he does. Soon after, she meets with the leader of the therapy group...and he deduces that she might just have murdered Ken's wife. But before he can go to the police, Lorraine kills him as well! Next, she notices that another woman is interested in Ken...and perhaps she'll have to kill her next!
The plot problems are that Ken clearly SHOULD have called the police and why he didn't isn't really clear...and it's a definite plot hole. The same with the therapist...he really should have played this better considering he believes the woman is a murderer!
On the positive side is the exciting ending. When Barnaby hears screams, be runs through the locked door and confronts the knife-wielding woman! He's so cool!