Sara Martinez is a hacker. She recently broke into the New York City foster care system to expose her foster parents as cheats and lawbreakers. However, instead of being hailed as a hero, Sara finds herself facing years in a juvenile detention facility and banned from using computers for the same stretch of time. Enter Mother, a British spy who not only gets Sara released from jail but also offers her a chance to make a home for herself within a secret MI5 agency. Operating out of a base in Scotland, the City Spies are five kids from various parts of the world.

Charlie Thorne is a genius. Charlie Thorne is a thief. Charlie Thorne isn’t old enough to drive. And now, it’s up to her to save the world.... Decades ago, Albert Einstein devised an equation that could benefit all life on earth - or destroy it. Fearing what would happen if the equation fell into the wrong hands, he hid it. But now, a diabolical group known as the Furies are closing in on its location. In desperation, a team of CIA agents drags Charlie into the hunt, needing her brilliance to find it first - even though this means placing her life in grave danger.

On a family trip to Paris, Greg Rich's parents disappear. They're not just missing from the city - they're missing from the century. So, Greg does what any other 14-year-old would do: He travels through time to rescue them. Greg soon finds out that his family history is tied to the legendary Three Musketeers. But when he meets them, they're his age, and they'll only live long enough to become true heroes if he can save them.

Ben Ripley may only be in middle school, but he's already pegged his dream job: CIA or bust. Unfortunately for him, his personality doesn't exactly scream "secret agent". In fact Ben is so awkward, he can barely get to school and back without a mishap. Because of his innate nerdiness, Ben is not surprised when he is recruited for a magnet school with a focus on science - but he's entirely shocked to discover that the school is actually a front for a junior CIA academy.

Having assumed the identity of a young D’Artagnan, Greg Rich is beginning to get the hang of things in 1615 Paris. But he hasn’t figured out how to get home yet. Or how to defeat the dangerous Michel Dinicoeur, who has made it his mission to destroy Greg, Athos, Aramis, and Porthos. When Dinicoeur escapes the Bastille and flees to Spain, the Musketeers charge after him, only to be ambushed. Someone is anticipating their every move. Could there be a traitor in their midst?