Mason is not imaginary. He's not a ghost, either. And he's most definitely not a hallucination. Mason is an Aerling, and the Sentinels' number one target. Separated to keep each other safe and alive until Mason's eighteenth birthday when Olivia is expected to guide him back to the world of the Aerlings, neither one was prepared to be stripped of their best friend, of the person they love most. The pain being away from each other causes is the least of their worries, though, as the Sentinels intensify their search for Mason and bring the threat of danger to a whole new level.