The setting is Bishop's Lacey, a small, sleepy English town in 1950. Flavia Sabina Dolores de Luce, eleven, is better at chemistry than most of us can probably ever hope to be. (And the Dolores is a lie. She sometimes fabricates things.) She is our narrator and when asked what her passion is in life, Flavia unhesitatingly says chemistry. She even has her own lab which was originally created by a great-uncle and then used by her mother, who died in Tibet when Flavia was still a baby. It's her sanctuary in her her family's sprawling and somewhat dilapidated manor house where Flavia lives with her reclusive father (Colonel de Luce), two sisters (Ophelia and Daphne, whom she loathes), and Dogger (Colonel de Luce's once-manservant/driver and now gardener with a loose grip on the day-to-day, as he came back from the war a bit shattered).
The novel starts on a seemingly normal day (after a fight with Ophelia, Flavia stole her lipstick, melted it down to mix with the essential oil of poison ivy leaves, and reformed the lipstick using a .45 caliber slug) and then something peculiar happens. The housekeeper opens the kitchen door to discover a dead jack snipe with a stamp impaled on its beak. Flavia's father is visibly shaken and later, Flavia eavesdrops at the keyhole of his study to see and hear him argue with a redheaded man Flavia has never seen in her life. For her quiet philatelist of a father, this is uncommon behavior -- but perhaps more uncommon is when Flavia walks through the cucumber patch in the pre-dawn hours and discovers a body. It is the redheaded man with whom her father had been arguing and he breaths his last word into Flavia's face: "Vale!" Unlike most eleven-year-olds, Flavia does not run in fear: "I wish I could say I was afraid, but I wasn't. Quite the opposite. This was by far the most interesting thing that had ever happened to me in my entire life."
Well, I think you can guess by this point that Flavia is not the kind of girl to simply leave this matter to the authorities, particularly when she calls them and once arriving, they tell her to run along. While the police conduct their investigation (in which her father is the prime suspect), Flavia does some sleuthing of her own. The result is a fantastic journey into her father's less-than-innocent school days, priceless stamps, and magic tricks that go horribly right.