THE GESTAPO officer is ruthless and sadistic; when he captures the beautiful blond agent he clicks his heels lightly and says: "So, we meet again." The British spymaster is suave and a touch Old Etonian; his eyes are half- closed, and sometimes there is the faintest suggestion of a weary smile at the corner of his mouth. The deputy Gestapo officer is bland and podgy. The young American major (Yale, Harvard Law School) has ash-blond hair, a slightly broken nose, and noble scruples about the morality of Allied intelligence operations. (Hence the spymaster's weary smile.) The French Resistance man is tall, with thick auburn hair and a mischievous half-smile, although his eyes have a quiet sadness. Churchill growls. Hitler pales with anger. Yes, foks, it's D-Day again.

The characters are by far the least successful part of Fall From Grace, which is a pity, because the events that form the framework of the book are true, and truly heroic. As a Collins general says, the Normandy invasion was "a damnably difficult proposition." Success or failure depended not so much on the initial landings as on "whether or not Hitler and his generals make the right decisions at the right time . . . the evening of D plus 2 . . . is when he must make the decision to go the whole hog and risk everything on throwing us out of Normandy. Before that, he won't be sure Normandy is our main effort. If he waits too long after that, it'll be too late." This was one of the great military gambles of history. The German 15th Army, heavy with armor, was covering the Pas de Calais area: the shortest and most obvious route. If Hitler ordered its seven Panzer divisions to Normandy while the Allies were still struggling, a German counterattack might well kill off the invasion attempt in its first week. And if the Allies got thrown out of Europe, would they try again?

The whole thing depended on persuading Hitler to keep his 15th Army away from Normandy. This was done by mounting a huge and complex deception campaign. British intelligence had been so successful at intercepting enemy agents and turning them around that by 1944 it ran the entire German spy system in Britain. Disinformation thus fed back to the Abwehr included details of a vast, completely fictitious force assembling in southeast England, opposite Calais. The best of the double agents was a Spaniard codenamed Garbo. The Abwehr so trusted him (and his 14 sub-agents and 11 contacts, all invented) that they awarded him the Iron Cross.

ALL THIS Collins describes thoroughly. Too thoroughly. He has not spared himself in his research and he does not spare us, either. It may be useful to know that Hitler never drank coffee, but does it help to be told that the coffee he never drank was brewed from a special Yemeni bean, sent via Istanbul and Kiel to Berchtesgaden, just before Christmas? Collins tells us everything, often more than once, and it makes for a long book. Every noun must have its adjective: "Cavendish lit a French Gauloise cigarette from the package in his khaki uniform blouse and blew two long streams of smoke from his flaring nostrils." There are dire signs of Fine Writing: ". . . the channel's turbulent cross-rips and races were gentled this April morning by spring's soft hand." There are ludicrous similes: a girl with a cigarette "gasped after her first draw with the frenzy of an asthmatic struggle for air in a crisis." To make a novel from history, Collins has grafted onto the D-Day deception plan a complicated drama involving the startlingly lovely blond agent, a German gun battery dominating the Straits of Dover, and much cheating and swindling by (and of) competing Allied intelligence agencies. Sex and torture occur from time to time. In the end the good guys win by the wrong means, as we always knew they would.

Half this book is documentary, half fiction; there is no doubt where Collins is more at home. His characters exist to tell the story, rather than to be the story. Curiously enough he misses one of the best jokes of the whole affair. Garbo helped persuade the Germans to keep their Panzers at Calais for six weeks; Normandy, he kept telling the Abwehr, was just a diversion. When the truth became inescapable, London assumed the Germans would sack him. Not so. The Abwehr persuaded themselves that the only reason the Pas de Calais attack never took place was that the Normandy landings were unexpectedly successful. Their trust in Garbo was unshaken. They still took his reports, and still paid him. He earned some 20,000 in all.