The Boggart and the monster - Susan Cooper

Emily and Jess Volnik have returned for another Scottish adventure, joining their old friend Tommy and Mr Maconochie, the new owner of Castle Keep, on a trip to Loch Ness, where a new expedition is determined to find the fabled monster.

The fun-loving Boggart - a mischievous shape-shifting spirit who has lived in the castle for centuries -comes along for the ride, but this time the Boggart has a serious mission. His cousin Nessie is trapped in the monster shape he took on long ago, and it's up to the Boggart to keep Nessie from being discovered by the expedition's high-tech equipment. Is modern science any match for the Boggart's ancient magic?