Agent Darcy is an agent-in-training at the Bureau of Sneakery.
Ninja Steve is an ninja-in-training at Ninjastoria
When we first meet Darcy, she is thirteen years old and is an exceptional student at the prestigious Agent school but she has of late been plagued with a string of second-bests to her rival, Agent Serena, and now the worst thing she could imagine has just happened. She failed a test. That makes her certain she will never be given a real mission and will never leave the "hidden territory that belonged to the Bureau" where she has spent her whole life so far.
Our first introduction to Ninja Steve comes on the morning of his twelfth birthday and yet another chance for his older sister, Ninja Nora, to prove yet again that she is way, way better at being a ninja than he is, resulting in a birthday cake to his face. He knows a good part of the problem is that he really, really, hates studying and would rather be out doing it than reading about doing it, even if the reading part is needed to understanding the doing.
The two will meet when Agent Darcy is sent on her first mission (her fears of never getting one having proven unfounded) as an exchange student to Ninjastoria where, in addition to being an actual student there for six months, is given a secret assignment to complete. Ninja Steve will find that helping her, besides being a pleasure because she is really pretty and a whole lot smarter than he is, will help himself become a better ninja.
They will also find that there is something very sinister going on which involves the nasty and very dangerous mecha-moles from the not quite far enough away Mole Republic, ruled by the mean Mole Emperor (not sure how an emperor is ruling a republic).