MI-5 is in trouble. All of their adult agents have been exposed. Luckily they've got a group of inner-city high school kids that secretly work for the British government. Welcome to MI High. Their top secret missions are given to them by the school janitor, who is actually an ex-MI9 agent who operates out of an HQ setup in the school's basement.
At the beginning of series 3, we find out that Daisy, Blane and Lenny have been set away to work in another country.