Shirley Deblois is an eleven-and a half year old girl who lives in an ordinary suburban house on an ordinary suburban street named White Rock Avenue in an ordinary city. Her family, however, is not ordinary. They're a family of eight living in their modest home, they're traditional Catholics and they homeschool.One day, Shirley delivers cookies to her elderly neighbors Jim and Janice only to find out that Jim's mower, Old Lucille, is missing. Who would steal Jim's mower? What malevolence has entered their quiet, ordinary neighborhood?Shirley embarks on her first ever detective case using the skills and knowledge acquired from her various homeschool courses, the family's large, eclectic (though sometimes outdated) home library and her father, a former policeman who helps her navigate her way through "the biz."She begins gathering information from her quirky neighbors, such as Crazy John who drives a lawn mower because the law won't let him drive a car anymore, Sonny who takes his dandelion problem very seriously and Pauline, the neighborhood tinkerer and good friend of the Deblois family.Shirley's list of suspects is long. There's the relentless Shaun of Shaun's Lawns, a for-hire lawn maintenance specialist that wouldn't mind Jim, a prospective customer, never getting his mower back. Old Mary who has had a long-running feud with Jim and Janice. Or Young Old Mary, who has always had the number two lawn in the neighborhood to Jim's number one.Shirley is in a race against the growth of Jim's grass which, when too high for suburban standards, will force him to buy another mower. Will Shirley get her school and chores done in time to work on the case? Will she find the mower? Will she find the culprit? Will she ever admit that she is not yet "almost twelve" but still technically only eleven and a half?