NOTHING QUITE EQUALS lying in the summer sun - having already paid your dues to the multinational corporations for exorbitantly priced gasoline, ridiculously priced motels and, if things continue as they have, beach sand itself that costs money - and reading a good book about one man alone fighting the insidious system.

So consider this man, Nicholai Alexandrovitch Hel, hero of Trevanian's new entertainment, as he confronts for the first time the hatchet man of the megalomaniacal - and successfully so - Mother Company. (Mother's man is named Jack O. Diamond, Trevanian never having been terribly subtle with names.)

Diamond is dispatched to stop Hel from using his unique prowess to halt the PLO from skyjacking a Concorde. As distasteful as the Mother Company (a consortium of the multinational oil interests) may find the Arab terrorists, they must be coddled to keep the Arab petro-states happy.

Trevanian sums up the confrontation thusly:

"Diamond's role was ludicrous: the Tom Mix of big business facing a yojimbo with a garden. Diamond possessed the most extensive computer system in the world; Hel had some file cards. Diamond had all the governments of the industrialized West in his pocket; Hel had some Basque friends. Diamond represented atomic energy, the earth's oil supply, the military/industrial symbiosis, the corrupt and corrupting governments established by the Wad to shield itself from responsibility; Hel represented shibumi , a faded concept of reluctant beauty. And yet, it was obvious that Hel had a considerable advantage in any battle . . . "

Yes! Though Hel is the central figure in a book marred by a cast of caricatures and obvious plotting, he is one of the most interesting fantasy figures to appear in recent thriller fiction. To the considerable extent that Shibumi is a character study of Hel, it is one hell of a pleasure to read.

Trevanian obviously set out, albeit with tongue in cheek, to create a superman to oppose the supersystem.

Hel was born in Shanghai sometime during the 1930s (a bit of vagueness the author doubles for mystery), the son of an incomparably aristocratic exiled Russian countess and a rather tedious young Prussian she would never deign to marry.

He grows up speaking five languages only to become, at the beginning of World War II, the virtual foster son of a Japanese general. He spends the years of the war in Japan studying the tactics and mind-set of Go, the oriental board game that is to Western chess, as Hel puts it, "what philosophy is to double-entry accounting."

He also becomes obsessed with the origins of that "ineffable quality" shibumi , which "has to do with great refinement underlying commonplace appearances . . . In the Personality of a man, it is . . . authority without domination."

After the war, much to his distaste, the dictates of survival force Hel to work for the occupying forces as a translator. In the meantime he somehow teaches himself the skills of Naked/Kill. This novel form of offense/defense uses ordinary household objects - a folded piece of paper , a drinking straw, a comb - as lethal weapons. Hel subsequently monsters as well the use of lethal weapons as lethal weapons, not wanting to be caught short.

There seems no end to Hel's masteries. He rises to the fourth level of sexual expertise. (It is said his skills can ruin a woman for life since she can never again hope for such an exerpience). He achieves world world-wide prominence as a spelunker. (The book's crucial action occurs in caves.) He picks up a couple more languages, including the incomparable complexities of Basque.

He is endowed as well with the intrinsic qualities of a mystic and a "proximity sense" that allows him to feel the presence of other people, including the nature of their concentration, whether he sees them or not. For comic book readers, Spiderman's "spider sense" is a comparable analogue.

Hel is imprisoned for three years by the Americans in Japan for an act it would have been unconscionable for him not to commit and unfair to reveal here. After his release he grows up, as it were, to become the assassin's assassin, a freelance terror of terrorists, hiring out to the highest bidder - or to friends for free.

Detailed gut-wrenching action has been the cornerstone of Trevanian's previous books - The Eiger Sanction, The Loo Sanctions and The Main . But here Trevanian leaves most of that potential action to the reader's imagination without diminishing the pace of the story. He makes it part of a past that Hel regards as relatively insignificant in his quest for the aesthetic sublimities of shibumi .

If only the other characters were more fully realized, there would be little to fault in this thriller. But basically, whether they are CIA operatives or Arab terrorists in training, Basque innkeepers or adventurous poets, they are paper-thin personalities to which the author constantly condescends, paradoxically, because they are so transparent and obvious.

Such hauteur sits well with Hel, but not with the author, whose arrogant tone seems directed as much at the reader as anyone else, and is often less amusing than tedious. The language and multilingual references drawn from obvious research tend to inure rather than impress.

Nevertheless nothing can diminish one's admiration and envy for Hel, and the reader is left hoping - as the last page is turned and the volume stowed in the beach bag - that Hel may soon be met again conquering another time, in fantasy, the unconquerable system.