The Mystery of the Invisible Spy is the tenth book in the “Adventure Island” series that Helen Moss wrote.
The book starts when Scott and Jack Carter arrive at Stone Cottage, the home of their Aunt Kate. Their friend Emily and her dog Drift, have already discovered a person that she thinks is a spy! The boys don’t believe her, especially when she tells them that the “Spy” drives a silver Ford Fiesta! Jack and Scott say that spies should drive Ferraris or Aston Martins!
When the man dresses up as a birdwatcher and goes into one of the birdwatcher huts, on the moor on Castle Key, the friends go into another hut to keep watch on him. In the hide the children watch a Skylark singing a song above its nest. When Scott looks in a logbook inside the hut, where it says “Superb pair of skylarks with late brood in nest. Otto and Karin, Frankfurt.“, he gets an idea. He thinks if the man really is a birdwatcher he would have noted the skylark display in the logbook. When he looks in the logbook in the other birdwatcher’s hut, he discovers that the “spy” had not written anything in the logbook!
Another day, when they were buying ice-cream, Jack notices that the ice-cream man is the same man that disguised as a birdwatcher! When Emily starts writing in her notebook about Operation Skylark and heads the page “Ice cream spy” they are shocked by the spy, who seems to be watching an old house that is being renovated and is meant to be owned by a retired gardener, leaning over them. Emily tried to hide her notebook but it was too late. The man had seen it.
After thinking for a while, the man who apparently is called Rudge, tells them that he is an MI5 agent! He also tells him that his mission is to keep an eye on the “retired gardener” who he says is a KGB agent!
When the children see bullet-proof glass and CCTV cameras being unloaded at the gardener’s house, they’re certain there’s more to him than meets the eye, but just how much can they trust Rudge, who is starting to do some very suspicious things?