UNDER COVER OF DAYLIGHT By James W. Hall Norton. 288 pp. $16.95

"Under Cover of Daylight" is an exciting, well-crafted first novel that smoothly places complex, believable characters into an action-filled plot.

The protagonist Thorn was an infant, just 20 hours old, when a drunken driver forced his parents' car off the road, into Lake Surprise. Both Thorn's parents died in the crash. Nineteen years later, Thorn drowned that drunken driver in that same lake, and in the 20 years since he took his revenge he has lived in a one-room shack on Key Largo, looking out at Blackwater Sound and a blinking channel marker, making a living tying flies for bonefish, and trying to overcome his guilt.

Both Thorn's way of life and his character are reminiscent of a young Travis McGee: He is part philosopher, part rebel, very convincingly self-contained. The south Florida setting is both integral to his character and descriptive of it. When his parents were killed they were driving him from the hospital "back to the Keys so he'd be officially, by local custom, a Conch. To give the boy roots. Maybe roots was wrong. Suction was a better word. The island didn't grant much purchase. Limestone and coral just under the couple of inches of sandy dirt. It was just a long, narrow strip of reef really. And with a little melting at the North Pole, one good force five hurricane, it would be reef again. But Conchs had suction. They could hold on to places where no roots could burrow in."

And so it is with Thorn. He has attached himself to his island world -- and more recently, to Sarah Ryan, an attorney with whom he has become involved.

Sarah is working with Thorn's foster mother Kate, fighting the real estate developers who are overbuilding the Keys. By using "blind endorsements, parent corporations, anonymous trusteeships, mock subscribers," Kate and Sarah have remained unknown while buying up various developments and shutting them down. But for one particularly ambitious project, "a small city" called Port Allamanda, they have made their fight public, raising the issue of an endangered species to prevent construction of more than a thousand condominiums, a "complete shopping center, banks, a couple of marinas, a golf course."

Simultaneously, Kate and Sarah have been smuggling marijuana, accumulating nearly enough cash to exercise their option to buy the land on which Port Allamanda is to be built. Then Kate is killed -- raped and murdered on her charter fishing boat -- and Thorn is drawn into a complex web of events as, with the help of his policeman-friend Sugarman, he tries to track down the killer. The track leads to Key West and back to Key Largo as Thorn pursues a psychopathic murderer-villain and the land developer who set him onto Kate. Along the way sudden violence erupts. There are car bombs, bloody fishhooks and fillet knives, silenced automatics.

While all this may sound a bit complicated, the plot is held together by the finely drawn relationship between Thorn and Sarah. Sarah, it turns out, is the daughter of the man Thorn drowned 19 years before, and she knows that Thorn killed her father. Although initially she tracks Thorn down to seek her own revenge, once Sarah gets to know him, her resolution wavers; and once he finds out who she is and her secret intention, the love-hate between them propels the story.

While each is wary of the other, they are also bound, as Thorn imagines it, "by thin, invisible umbilicals, hooked together. Every pinprick made the other wince. Her stricken face, the panic in her eyes at what she might do, was a replica of his face. Her exhalation came with his inhaling. Twined. Something more powerful than love binding them."

And that something more -- the internal forces that lead to the resolution of their desperation and confusion -- is what makes this novel so pleasingly deceptive: Within the context of a fast-paced mystery, author James W. Hall is able to explore themes of vengeance, love and forgiveness. In the process we are given an insider's view of the small-town quality of life in the Florida Keys and the assortment of eccentric characters who inhabit them.

This accomplished novel should not be missed. The reviewer is the author of a novel, "Profit and Sheen.