Erik Morissey Ganger, famed explorer and detective and his mischief-making sidekicks, twins Sadie and Saskia Dopple, didn't go looking for a secret tunnel beneath the school.

They never intended to make the acquaintance of a shifty private eye with a nose for trouble.

It wasn't part of the plan to come face to face with an old enemy, one with an agenda of his own that could destroy them all. And unravelling the 'secret of indigo moon' was the farthest thing from their minds.

Erik, Sadie and Saskia plunge headlong into a new and perilous mystery, one that challenges everything they thought they knew about their lives, themselves and who it's safe to trust.