Whether Cleo is diving for the ball on the soccer field or dangling over a pool of man-eating sharks, she’s always ready for action. She’s a good friend, an amazing athlete, a natural leader, and a fearless adventurer. Sometimes a bit too fearless, if you ask her friend Evan. Cleo sometimes lets her curiosity lead her down dangerous paths...

There is only one thing Evan likes more than reading, solving puzzles, and playing his trombone—telling jokes. Do you know why an elephant uses his trunk as a bookmark? So he always nose where he stopped reading! Jokes help calm Evan down in tense situations. But although he may be a bit timid at times, he’s the kind of friend you can count on when the going gets tough.

Cleo and Evan don’t know how Ms. Barbara Crowley came to be the librarian at their elementary school. All they know is that she isn’t very nice…and she’s awfully mysterious. Why does she always wear an old-looking brass key on a chain around her neck? And why do Cleo and Evan often hear her clicky heels tapping a path to a rarely-visited section of the library? There is only one way to find out—follow her through a secret passage to a hidden library beneath the school!

At sixteen-years-old, Artie Doyle is the youngest detective with the London police department. And he has been given the biggest case of his life—the disappearance of the famous Musgrave Moonstone. But since Artie is a little…scatter-brained, he’ll need Cleo and Evan to help him. Their first clue? The moonstone is really a huge diamond!

Artie Doyle’s English bulldog Watson would rather take a nap or snack on biscuits than investigate a crime. But when he picks up the scent of a clue, nothing can stop him—or his drool!

Avery Phillips is the chief of an international spy organization known as A.C.R.O.N.Y.M.—The Agency for the Capture and Research of Nefarious Masterminds. He knows the ‘Y’ doesn’t stand for anything, but he doesn’t care. The only thing he cares about is rescuing his top spy from the clutches of a super-criminal known as the Viper.