The Dark at the End of the Tunnel : THE INNOCENT <i> by Ian McEwan (Doubleday: $18.95; 270 pp.) </i>

Something there is that does not like a tunnel. It is the theme of Ian McEwan’s provocative though erratic novel of espionage, set in Berlin during the early intensity of the Cold War.

A novel of espionage, that is, with spying as metaphor for a larger type of human isolation. Graham Greene used the thriller genre in a similar way, and although there are one or two aspects of “The Innocent” that may suggest John Le Carre, a closer comparison is to such Greene entertainments as “Brighton Rock” and “The Confidential Agent.”

In his non-entertainments, such as “The Child in Time” and “The Cement Garden,” McEwan, who is English, wrote about people living tightly on the verge of disintegration. The verge is society’s, too; McEwan’s human and social textures have the translucence of skin stretched over infection. The skin rips unpredictably, but the rips are tiny and the poison leaks without really draining.

“The Innocent” is more spectacular; its rips are volcanic eruptions, though the result is not purgation but parody. It is not as serious as its predecessors, though it is more fun and likely to be more popular. It stretches the nerves. It does not stretch the mind so much as pluck at it.

McEwan bases his story upon a real event. In 1955, the American and British intelligence services built a tunnel into East Berlin to tap the main East-German and Soviet telephone cable. The tap worked for a year until the Russians discovered it. In fact, they seem to have known about it all along, thanks to the work of a British mole, George Blake.

McEwan brings in Blake as a peripheral figure, though significant to the plot; he mentions one or two other real people, and he uses a lot of real detail. The story and its main characters are fictional, though. The fiction is a tight weave of spy story, love story and parable; along with black comedy that goes from wild as an adjective, to wild as a verb; as New York’s Central Park muggers used it: as Doomsday assault.

Using a classic device, McEwan introduces Leonard Marnham, his innocent, into an infinitely non-innocent world. By the end, this human tabula rasa will not only have registered every kink and whorl in this world, but go on to out-kink and out-whorl it.

Leonard is diffident, polite and virginal. He lives with his parents in a semi-detached London house, and works as a technician in the research laboratories of the British telephone system. Suddenly he finds himself transferred to Berlin, awestruck at having a two-bedroom apartment, a job assembling and servicing taping equipment in the tunnel project, and the patronage of Glass, a hard-charging CIA operative who is his boss and mentor.

One kind of virginity is ruptured as Leonard revels--though shyly--in being a secret agent and a “conqueror” in a Berlin that is still war-shattered and subservient. Another kind goes when he meets Maria, a young German woman who works for the British occupation forces.

Maria is not as innocent as Leonard. She suffered under the Russian invasion and under a brutal husband whom she divorced, but who still turns up to abuse her. She has an edge of hard-won practicality, but she is gentle, as well, and Leonard is a lover with whom she can take the initiative and not be frightened. Burrowing under layers of blankets in her tiny, freezing apartment, they are two furry moles, tender, passionate and snug.

It is a tunnel, though, as well: hidden, cut-off and generating its own peculiar passions. If Leonard’s secret identity allows him to imagine himself as Berlin’s conqueror, his seclusion with Maria brings out a spark of sadism. It is brief and tentative--Maria walks out for awhile, reaching the tunnel, in effect--and he is utterly ashamed of himself.

As for the spy tunnel, it nourishes other kinds of baleful fantasies. Glass personifies the illusion of a secret allied advantage over the Russians; though at the end, he emerges as oddly sympathetic. And Leonard is recruited by the senior British agent to spy upon the Americans, who are the abrasive senior partners in the project.

In a wonderful introductory scene, Glass initiates Leonard into the world of clearance levels. Level 1--the public level: The building from which the tunnel operation starts is ostensibly a military warehouse. Level 2--for the building contractors: It is really a radar installation. Level 3--for the true initiates, the technicians digging the tunnel and operating the equipment: It is the telephone tap.

Glass is immensely proud of these layers of knowledge--a tunnel within a tunnel within a tunnel. Is there a Level 4? If there is, Glass says, he doesn’t know about it. But of course, there is; it is the British subplot. And this subplot will eventually allow Leonard to outwit all four levels when his tunnel life with Maria goes grotesquely wrong, and in a fashion, save them both.

McEwan’s account of the building and installation of the communications tap is taut and exciting. Much more remarkable is his skill in using the tunnels--real and metaphorical--as instruments of both moral suspense and black humor. His tunnels, in fact, are like the doors in French farce; they switch us comically between revelation and mystification.

Other things are less successful. The story of Leonard and Maria, darkening steadily, gets out of hand when her husband appears. There is a brutal fight, a killing, and 12 unsparingly detailed pages about the dismemberment of a body. It is brilliantly done, in its own way, but it is all but unbearable to read.

More seriously, it is like a power surge that blows out the wiring. The morality farce takes over again, but we may be too jangled to appreciate it. Furthermore, after so much tension, the ending seems lackadaisical and routine. An epilogue, set years later, turns this routine ending into a “happy” ending that is utterly banal. It is as if McEwan, having set up his complex and powerful device, had lost both control and interest.

“The Innocent” provides a series of strong and often subtle sensations. But there is something about them that resembles not so much the experience of a story as the kind of pseudo-experience administered by electrodes attached to nerve receptors. The taste is indistinguishable from the real thing, but the aftertaste is flat. “The Innocent” evokes a dark moral world in a highly entertaining fashion. Unlike Greene’s entertainments, however, McEwan’s leaves not even the trace of a feeling behind it.