Recovering drug addict Tommy Jarret rents a Scottish chalet to check out reports of a sea monster near the dour village of Drim. But when he is found dead of an apparent drug overdose, Lochdubh constable Hamish Macbeth suspects foul play.

Deciding to pose undercover, Hamish infiltrates the illicit drug trade in nearby Strathbane, but his daft scheme springs a leak when he is teamed with a tough Glasgow detective inspector named Olivia Chater, a.k.a. "concrete knickers." Now, in a charade that puts the intractable Highlander in a designer suit among the sharks of the underworld, Hamish's craving for justice will be tempered by a dependence on Olivia. For the lanky lawman, it's time to sink or swim—and it would be equally dangerous to flounder or fall in love.