A stay in a remote farmhouse in Wales should be a quiet and peaceful holiday for Philip, Dinah, Jack and Lucy-Ann. And indeed, there seems little likelihood of any new adventures cropping up as they set out on a camping trip on donkey-back in search of the beautiful Vale of Butterflies. But when they become lost in the thick mountain mist, strange things start to happen. Before too long, the children discover a mountain stronghold that hides a bizarre and dangerous secret experiment — one whose perpetrators will stop at nothing to protect their sinister activities.