You know the drill. In an effort to win favor with the local kids in a new town, our eleven-year-old has to perform some stunt in order to 'join the gang.' This stunt is usually designed to be scary to the person in question, so he has to climb a tree (he's afraid of heights) and break into a house in order to steal (for a day or two) a particular cat. And of course, the owners are home.
However, in this case, the man has a literally murderous temper, and our protagonist Robin, while grabbing the cat, hears and sees a few things he shouldn't, and is spied in return. He runs away, and because he is nervous about the relatives he is staying with, says nothing, leaving the murder to be blamed on an "intruder." The rest of the novel is an exercise in suspense to see who will be discovered first, Robin or the murderous husband.
As with many of Marian Babson's "mysteries", there is no real mystery to be solved. She just ups the tension in little notches, setting up near misses and misconceptions and interior monologues of the murderer until you realize that there's no way the two cannot meet, and one can only hope that there are police at the scene when the final meeting does occur.