In Kevin Hwang’s new novel, The Regression Strain, Peter Palma signs on as a shipboard doctor on a cruise, hoping he’ll get a much-needed change of scene and some distance from his personal problems, while he helps passengers with sunburn and seasickness.

But unfortunately, he’s not about to get any of that in this thriller.  Almost immediately, Peter notices something’s a little off on the ship. Passengers seem a bit volatile, a bit too stressed for vacationers, and his new boss seems a little strange, almost like she has no filter. It’s the beginning of a twisty medical thriller.

Plus his little sister shows up as a passenger on the cruise, so poor Peter’s not even going to get any space and solitude to face his personal issues!

The Regression Strain isn’t exactly a locked-room mystery, but it had many of the elements I enjoy in locked-room mysteries. Because almost the entire book takes place on the ship, there’s no ability to call in police when passengers turn violent or to send very serious patients to the emergency room. Sure, Peter can call and email other physicians, but he’s largely on his own, making high-stakes decisions as best he can. 

There’s an early-covid vibe when passengers come in to the clinic with similar, but not identical symptoms. Peter just knows something’s up, especially when more and more patients have no filter. I liked this a lot, especially as Peter and the nurses, Mandy and Luisa, keep trying to respond to inappropriate comments and reactions in a professional, reasonable way. This conflict between animal instincts and social norms comes up again and again in this thriller. As the story goes on, Peter keeps patching up injuries as the result of personal violence. Not just the odd bar fight, but relatives attacking each other. And this whole time, there are other passengers stopping in with unrelated medical issues. It’s a busy clinic, even as the mystery disease spreads! Peter tries to keep working pretty extreme conditions, which works to develop his character.

The mystery illness attacks the brain, causing people to lose their self-control and leading to outbreaks of violence and general unhinged behavior. A few scenes in this novel are gross, but it’s not to be shocking or edgy, it’s more about the mystery and about using violence to express questions about human instincts. We all get angry, don’t we? So what keeps us from strangling an annoying relative or stabbing an unpleasant supervisor? Or, for the less anger-motivated, what keeps us from groping a cute coworker? 

Without spoilers, since this is the kind of novel where you want to guess and discover along with Peter, I can at least say there’s a villain and the villain has reasons that make logical sense to him. (That’s always the most satisfying baddie for me.) Are we better off with modern society, or were we as a species better off in the past? Would everyone be happier if we just went with our animal instincts?

This medical mystery is set up well, so of course I wanted to know how the disease worked and how it got on the ship, and there are loads of reveals and twists, right up to the end.  But overall, I most enjoyed the scenes when Peter is trying to be a good doctor, under massive personal and professional strain.

The Regression Strain asks us readers to consider if people would be any happier just following their animal needs, without rules or restrictions, but Peter’s actions remind us that caring for friends and family and helping those in need is more important.