Callum's eyes snapped open. Something was wrong: very wrong.

He had been dozing fitfully by the embers of their small campfire. Just as he thought he might be getting off to sleep a strange feeling of emptiness roused him from his slumber.

Across the camp, Lena sat bolt upright. She glanced round the camp until her eyes finally settled on Callum.

"What on earth was that?"

"I don't know." Callum's face was etched with concern. "How do you ...