Katie Brunswick is an eleven-year-old girl who lives with her parents and two younger sisters in a small Canadian town. This summer, she's hanging out with her friends, an all-girl group who call themselves The Posse. Katie is a competitive swimmer and her other favourite thing to do is read mysteries. Imagine her excitement when a whodunit turns up in her neighbourhood.

Katie peeks through a dirty window in a deserted house and spies a body! While she's figuring out who was murdered and by whom, she tangles with some unsavoury characters. Meanwhile, she's saving animals from climate change... and trying not to argue with her two younger sisters.