THE TURNABOUT of Vitaly Yurchenko drives stunned observers to the full range of theories created to deal with the murk of espionage. Always a favorite is a theory of Soviet wizardry holding, in this instance, that the KGB cleverly planned the defection and redefection of one of its elite officials to embarrass the CIA or to weaken Ronald Reagan's summit hand. Another is that Mr. Yurchenko was caught up in the peculiar confusion of motives and roles to which -- if the spy novels have it right -- people in the business of deception are especially prone. A third theory is that in the last year or so a cataclysm in the whole system of international espionage has created among agents and intelligence services a pervasive sense of insecurity, of familiar moorings being lost, and has resulted in a series of defections and unmaskings that may not yet have come to an end. A fourth school holds that Mr. Yurchenko was not nearly so big a fish as was generally supposed when he was caught.

You do not have to be able to plumb the depths of this case on the Soviet side, however, to have disturbing questions about the manner in which it was handled on the American side. From the first exultant leaks to the press about the catch of a blue-chip defector, to the glee freely expressed in the resultant sure discomfort of the KGB, the CIA and those influenced by its briefings in Congress and elsewhere have acted in a strangely incautious and amateurish way. It is not clear that professional procedures to ascertain the bona fides of a defector, and to retain the confidence of this difficult breed, were followed closely here. Early on, according to what has been reported, Mr. Yurchenko enjoyed cozy meetings in a social setting with the CIA's brass. Somehow, a ranking Soviet officer still in the stage of debriefing was watched so laxly that he could make his way to a Soviet haven in Washington.

Mr. Yurchenko, in his press conference on Monday, had every reason to give a report that he thought might ease his passage home in what are bound to be severe circumstances. His observations on the way he was treated by his temporary American hosts have to be taken skeptically. People who do the work he chose can have no illusions about the unforgiving nature of the world they inhabit.

We understand that there are events and facts and relationships that have to be held secret in these matters. But Americans also need a reliable explanation of what happened in this apparently unprecedented case. They need to know how the CIA let itself be made a fool of in so incredible a fashion and how responsibility for it is to be assumed -- and by whom.