Forensic expert Professor Nick Fennimore has engineered lectures in Chicago and St Louis - a ploy to get to Detective Chief Inspector Kate Simms. She's in the United States on sabbatical with St Louis PD, and he's keen to see her again. Simms is working with a `method swap' team, reviewing cold cases, sharing expertise. But Simms came to the US to escape the fallout from their previous case - the last thing she needs is Fennimore complicating her life.A call for help from a sheriff's deputy in Oklahoma seems like a welcome distraction for the professor - until he hears the details: a mother dead, her child gone - echoes of Fennimore's own tragedy.Nine-year-old Red, adventuring in Oklahoma's backwoods, has no clue that he and his mom are in the killer's sights. Back in St Louis, investigators discover a pattern: victims - all of them young mothers - dumped along a 600 mile stretch of I-44. The Oklahoma and St Louis investigations converge, uncovering serial murders across two continents and two decades. Under pressure, the killer begins to unravel, and when a fresh body surfaces, the race is on to catch the I-44 killer and save the boy.