Olivia's best friend is not imaginary. He’s not a ghost, either. And she's pretty sure he's not a hallucination. He’s just Mason. He is, however, invisible. When Olivia spotted the crying little boy on her front porch at five years old, she had no idea she was the only one who could see him. Twelve years later when new-girl Robin bumps into the both of them and introduces herself to Mason, they are both stunned. Mason couldn't be more pleased that someone else can see him. Olivia, on the other hand, isn't jumping at the chance to welcome Robin into their circle. Jealousy may have something to do with that, but honest fear that Robin's presence will put Mason in danger is soon validated when a strange black car begins showing up everywhere. The race to find out what Robin knows in time to protect Mason from whatever threats are coming becomes Olivia's only focus.

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.