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.

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?

Charlie Thorne is a genius. Charlie Thorne is a renegade. Charlie Thorne isn’t going down without a fight. After tracking down incredible discoveries by Einstein and Darwin, Charlie is back. This time, the great ruler Cleopatra has left behind an extremely valuable and powerful treasure, its location encoded on an ancient stone tablet.