A young man caught up in a spider's web of international espionage and superpower politics is now languishing in a Moscow prison while President Carter and Leonid Brezhnev are trying to disentangle the hundreds of threads that lead to his cell. Carter has committed his personal prestige, and therefore that of the presidency, to the plain statement that the 29-year-old Jewish dissident, Anatoly Scharansky, was not working for the CIA as Moscow has claimed. Brezhnev appears willing to withdraw the claim, but the KGB does not want to let go of its quarry.

The arrest of Scharansky in March was provoked indirectly by Carter's human-rights campaign. Moscow was looking for ways to show that the campaign would do more harm than good, and it began arresting the dissidents for whom he had spoken up. There was little Carter could do about the arrests, but when Scharansky was accused of working for the CIA, the President could at least establish whether that was so, and he denied it. He has also made it clear to Brezhnev more recently that the trial of Scharansky on these charges could cause serious damage to U.S.-Soviet relations.

The Kremlin's reply last month, in the form of a Tass report, still insisted that Scharansky was guilty. But it no longer claimed, as did the original accusation, that Scharansky had worked for the CIA and had collected defense information. He is now accused only of passing to the West slanders to be used against the Soviet Union, and of providing it with information designed to disrupt East-West trade. Whatever the truth of theses charges, they are far milder than those contained in the original KGB scenario for a show trial that could be deduced from earlier articles in the Soviet press.

But the KGB has not given up altogether. "The traitor to his motherland," says Tass of Scharansky, "will be punished with all the severity of Soviet law." While Brezhnev wants to get back to business with Carter, the KGB hardliners want to make sure the dissidents can be kept under control. To the hardliners, this has always meant arrests, trials, deportation to Siberia. They have always believed that the best way to handle the dissidents is to punish them "with all the severity of Soviet law," and they have always said so.

The moderates in the leadership have argued that the best way to deal with the problem is to let the dissidents emigrate. While the KGB has its own bureaucratic interests to protect, that part of the Soviet establishment that has developed a vested interest in relations with the West would prefer to let the dissidents go. When Carter receives Foreign Minister Andrei Gromyko or Foreign Trade Minister Nikolai Patolichev, and brings up the subject of Scharansky when they want to talk about SALT and about the expansion of trade, his visitors can hardly be very pleased. They would presumably prefer to see Scharansky out of the Soviet Union instead of interfering with the orderly progress of the negotiations that are their primary concern.

When Brezhnev divided critics of Soviet society into three categories in a speech he made earlier this year, some of the Moscow dissidents welcomed the distinction he made, because they saw it as an attempt on his part to establish a more moderate policy. In the first category, according to Brezhnev, were those who criticized various aspects of Soviet life, but did so constructively - "We are grateful to them." In the second vategory were those whose criticism was erroneous - "those we regard as misguided." Only in the third category were the enemies of socialism - those who engaged in anti-Soviet activity and turned to "imperialist subversive centers" for support.

Some of the dissidents welcomed the speech because it gave them an opportunity to argue - or so they thought - that they belonged to the first two categories. If the KGB went after them, it would have to prove that they had worked for the "imperialists" before they could be sentenced. Perhaps they were overinterpreting Brezhnev's new definition, but they were not the only ones who paid close attention to it. The KGB chief, Yuri Andropov, also took up Brezhnev's formula, and found that he did not like it - perhaps for the same reason taht some of the dissidents liked it.

In a speech in September, Andropov said that Brezhnev had "set out clearly the party's position" on dissidents, and he then repeated word for word Brezhnev's description of the first two categories of critics: the constructive and the misguided. But when he came to discuss the third category, which Brezhnev had linked with "imperialists," Andropov expanded it to include a broader variety of political transgressors - those who "spread false rumors and try to organize all kinds of antisocial sorties."

As the Soviet union's chief policeman, Andropov wanted it to be clearly understood that the KGB would continue to suppress critics even if they had no obvious "imperialist" connection, whatever Brezhnev might say. Brezhnev might well be willing to let Scharansky go, not because he approves of dissidents, but in the hope that this would help to improve his relations with Carter. Andropov, on the other hand, would presumably argue that if the dissidents who are now in prison are treated too gently, this would make the KGB's job more difficult in the future.

What happens to the young man in the Moscow prison has little to do with the crimes he is alleged to have committed. But it has a good deal to do with relations between the Kremlin and the White House, and with Kremlin arguments on matters of high policy.