I've put it as 4 stars but I may well come back later and change that to 5. I really can't decide.

At the start of the story, this novel did not have "the magic". You know, the magic you feel when you pick up an exceptional book and can't put it down? In fact, you can't give your attention fully to anything, or anyone, else?
This book did not start off with that effect. But the effect did seep it's way inside about halfway through, and the second half of the book was exciting and unputdownable.
I found the story to start off being unrealistic, and I felt as though Ali Sparkes was pushing the storyline far too much. For example, had a witty line that she had thought of a few months ago and would make 10-year-olds laugh? No problem, just make the character say it even though it doesn't match the story or the character.
Also, it irritated me that Jack was a so-called Genius, and yet showed no signs other than being able to do maths.
However, these complaints faded as the book went on. The characters and storytelling improved, leading to a very dramatic and unexpected finish.
How else can I end this review than by saying that NOT ENOUGH PEOPLE HAVE HEARD OF ALI SPARKES.