Amidst Shetland’s fog-bound setting, Jimmy Perez is back in an elaborate murder mystery that skirts around fantasy and reality. Eleanor Longstaff, her husband, Ian, her best friend, Polly Gilmore, and Polly's boyfriend, Marcus, attend Lowrie Malcolmason’s “homefarin” at the Meoness community hall. But Polly--a librarian who specializes in folk stories, British myths and legends--has a sense of dislocation about the whole event. Considered cosmopolitan Londoners by the locals, Polly and Eleanor are old university friends, and Polly has been supporting Eleanor’s efforts to have a baby after a late miscarriage.

Polly begins to feel a new kind of stress and desperation when she spies a young girl dancing on the beach, dressed in white--a kind of old-fashioned party dress. The mist slides and the image blurs, then Polly sees an ice-like Eleanor looking as though she’s slowly melting. It’s easy believe in ghosts in a place like this where “all this history is so close to the surface.” The strange light of the “simmer dim” could almost be hallucinatory.

When Eleanor goes missing, Sandy Wilson, Jimmy Perez, and Willow Reeves jumpstart the investigation, Jimmy proving to be Willow’s reference point in these harsh northern islands. As the investigation fans out to encompass Eleanor’s murder, Cleeves's deft hand is clearly seen in the voices of Sandy, Jimmy, Willow, and Polly. The team learn that Eleanor was in the middle of a project, a film about ghosts and contemporary hauntings. Polly tells Jimmy that her friend was delighted over the ghost of Peerie Lizzie, a little girl who was the daughter of the owner of Springfield House and who drowned close by in 1930. Today Lizzie is supposed to haunt the land around Meoness. Lack of sleep and confusion about Eleanor’s death make Polly question her judgment, convincing her that she’s actually seeing Lizzie. Had she imagined the dancing child at the wedding party?

Ian’s grief and fury is unbearable. He tells Jimmy that Eleanor went “all theatrical and melodramatic,” accusing him of not taking her seriously and of thinking she was mad. There’s also Eleanor’s email to Polly the night of the “homefarin,” Perez repeating the words in his head: “Don’t bother looking for me. You won’t find me alive.” As the case unfolds, Willow tries to imagine the scene, the strange “half light” of early morning and Eleanor waiting and shivering, perhaps wrapping her cloak around her party clothes. Willow is convinced that Ian knows more than he’s letting on. He refuses to speak about Eleanor’s life in London and her difficult relationship with her mother, Cilla, a self-righteous snob who thought Ian was not good enough for her headstrong, independent daughter.

Attempting to win Polly’s confidence, Jimmy and Sandy become concerned that Polly, Marcus, and Ian are in danger, especially when another death is linked to Eleanor. At Springfield House, the epicenter of the investigation, new owners Charles and David have their own problems, desperately trying to keep their relationship intact by ignoring any unpleasantness, anything that might be difficult or uncomfortable. Charles is certain there’s some connection between the House and the ghost that Eleanor claimed to have seen. The fog, meanwhile, closes in, trapping Cleeves’s characters--chiefly poor, beleaguered Polly, who feels haunted by the light nights and the horror of all that has happened. Locked up in “this place in the beach” and obsessed with the memory of Eleanor, Polly increasingly unravels, shut in by the weather and the hill where her best friend’s body was found.

It's hard to believe that such gruesome murders are being committed in this isolated, beautiful setting where the weather-beaten locals and their hardscrabble existences are tenuously linked to the tourist dollars from big-city dwellers. In this landscape, certain inhabitants conspire to protect their children from the evils of visitors, hiding behind a veneer of small-island regularity. There are suspects of all types - from a woman called Monica, at first a hurriedly scribbled name in Eleanor’s notebook, to Grusche and George Malcolmason, Lowrie parents who spend an inordinate amount of time obsessing over their son. Even now, George, a retired lighthouse keeper, remains a mystery, sometimes strongwilled and always hiding the truth from Jimmy even at the risk of being charged with murder.

Like the previous novels, there’s a nice connection between Jimmy and Willow, the pair clearly attracted to each other but uncomfortable about taking it further. Jimmy is a dynamic and good man with a hard job navigating through an even harder time in his personal life. Smart, shrewd, intuitive, moral, and fair, Jimmy must deal with the survivors and family members of the dead along with witnesses and suspects, and he does it all with kindness, sensitivity, and respect. Jimmy welcomes Willow’s willingness to assist him whenever possible even when she becomes more than a little desperate, often clutching at straws with only Sandy, Jimmy’s loyal subordinate, to guide her.

Cleeves excels in capturing Shetland’s stark, harsh beauty: the windswept land, the gray churning ocean, and hovering clouds. The often gloomy landscape is the perfect setting for tragic death by murder. Always eerie and haunting (thanks to the ghost of Peerie Lizzie) and certainly mysterious, Thin Air is perhaps the best and most compelling in the series so far.