Early in 1968, the Soviet submarine PL-751 surfaced on its way to a station off Hawaii where it could lob its three nuclear-tipped missiles at Pearl Harbor if Cold War tensions, inflamed by Vietnam, escalated into war.

With its home port of Vladivostok far behind, the submarine had to recharge the batteries required for underwater operations. But an explosion suddenly sent the PL-751 and its crew of 97 to the bottom of the Pacific three miles below.While the Russians conducted a massive - but futile - search for the wreckage, the U.S. Navy's $16 billion undersea detection system pinpointed the site of the accident, leading to one of the most dramatic salvage efforts in history.

The Navy still won't confirm that the operation took place. But "leaks" over the past 28 years have established that a U.S. submarine recovered the PL-751's missiles, cryptographic machine and code books and the bodies of several Soviet crewmen.

Now, Roger C. Dunham, a nuclear reactor operator aboard the U.S. sub, has put a human face on the sensitive "mission impossible" in a compelling new book, "Spy Sub," published by the Naval Institute Press at Annapolis, Md. ($27.95).

Dunham had to tiptoe through a security minefield to win CIA approval for his book. He had to give his shipmates and their submarine false names. And he could not describe how they retrieved priceless intelligence from the PL-751.

But that part now has been fleshed out by reporters, notably Pat Sloyan, Washington correspondent for Newsday. He identified the sub as the USS Halibut, a former cruise missile submarine converted for covert activities.

The Halibut, dubbed the Viperfish by Dunham, found the PL-751 with its underwater camera near the end of weeks and weeks of exhausting underwater sweeps. Until then, the crew didn't know what they were looking for.

But when the ship's photographer saw the sub photos, he phoned Capt. Charles Moore to announce: "We have found what we were looking for." It wasn't a minute too soon. The Halibut was running dangerously low on nuclear fuel.

Dunham writes that the photos were stunning: "The main section of the submarine, lying on its side, showed up in stark relief from the surrounding mud . . . the superstructure appeared to have buckled from the stresses of its final descent."

Operating from a large area known as the hanger, as the Halibut hovered about 300 feet under the Pacific, civilian scientists assigned to the mission used giant claws to recover the Russian missiles, code books and cryptographic machine.

The Halibut's crew had been despondent over what they feared would be the failure of their mission. Now they were jubilant. "A feast was served, banquet style," Dunham recalled. "Punchbowls were filled with juices having a special tang."

But the crew still didn't realize the importance of their mission, even when the Halibut won a presidential citation and a letter from President Lyndon B. Johnson praising the sailors for "significant" contributions to their country.

Sloyan quoted Navy officials as saying their feat enabled U.S. military planners to read Soviet submarine messages for six years and to deploy U.S. anti-submarine forces against Soviet missile submarines.

"It was an intelligence coup," said John Caven, retired chief U.S. Navy scientist, who oversaw conversion of the Halibut.

The story didn't end there. In 1974, the CIA recruited the eccentric billionaire Howard Hughes and his Glomar Explorer recovery ship in an even bolder effort to raise the entire 2,800-ton Soviet submarine.

The Glomar succeeded in retrieving a 100-foot section of the submarine containing two nuclear-tipped torpedoes and the bodies of six Russian sailors. But most of the hull, weakened by the original explosion, was lost when a giant recovery claw broke.

This time, however, rumors began surfacing about the project. Then CIA director William E. Colby tried to offset them by giving curious reporters broad details of the effort on condition they not use the story unless someone else did.

Columnist Jack Anderson refused to go along and broke the story on his weekly radio program. The nation's newspapers followed suit, touching off a broad but inconclusive controversy over whether they were justified.

Dunham, a college dropout when he joined the Navy, attended the Los Angeles School of Medicine of the University of Southern California after his discharge and graduated in 1975. He now practices internal medicine in Santa Barbara, Calif.

The Viperfish, or Halibut, was decommissioned in 1976 as several former crew members "spoke eloquently of her past," Dunham said. Transferred to the mothball fleet, the sub was cut up for scrap metal in September 1994.

Meantime, veterans of the spy mission meet every five years, Dunham said, "to remember the days gone past and to pay tribute to their shipmates who have `rested their oars' and sailed on to their eternal patrol."