There's a new resident in Castle Key - and somebody is watching him! Scott, Jack, Emily and Drift soon suspect that the new resident is an enemy spy. It explains the bullet-proof glass in the windows of his house. And the MI5 agent who is clearly keeping tabs on him! But what is an enemy spy doing in Castle Key? Can the friends reveal his true identity? And uncover his top secret mission? Join Scott, Jack, Emily and Drift as they investigate a man so mysterious he's almost invisible!