Having just finished reading Karl Popper's The Open Society and Its Enemies: Plato, I must say that he makes a pretty damning case against Plato. However, I think it is plain that his defense of Socrates is very much in error.
His claim that since the most totalitarian and antidemocratic dialogs (Republic, Laws) seem to be among the later dialogs, this implies that they were written after Socrates' death, when Plato's philosophy was drifting from Socrates' more egalitarian ideas. Popper dismisses the "Socrates" character in the dialogs as Plato attempting to lend his destructive theories some weight of authority in a cunning plan to undercut and destroy the democratic leanings of Athens.
In the last chapter (Chap 10, "The Open Society and Its Enemies)" he gives some background to the greek culture wars going on during Socrates' and Plato's times: the rise of democratic (and imperialist) Athens, the cultural conflict between the other conservative greek polis such as Sparta and Thebes and the more progressive Athens, the progression of that conflict into the Peloponnesian war, the oligarchic coup that led to the bloody "Rule of the Thirty Tyrants" (many of whom were Socrates' students, including Plato's uncle Critias), the democratic revolt of Athenian democracy that overthrew the oligarchy, and the new democracy's sentencing of Socrates to his choice of exile or death on the charge of corrupting Athenian youth, which Popper interprets as meaning teaching many of the thirty that had overthrown democracy and murdered more Athenians than did the Spartans in the entire Peloponnesian war.
Popper then goes on to claim that Socrates' association with these was purely benevolent, that the apparent contrast between his "obvious" democratic leanings and their obviously (and murderously) antidemocratic leanings was due to his earnest desire to convert them to the democratic party.
This seems to me to be a case of pure wishful thinking. The critical elements here: the drift from democracy to tyranny that Plato's dialogs attributes to Socrates, the association of Socrates late in life with a large number of antidemocratic forces, and the resurgent's democracy's exile-or-death sentence on Socrates can most easily be explained by Socrates' own philosophical shift from egalitarianism to totalitarianism during the turmoil of the time (Thucydides mentions him several times in his account of the Peloponnesian war).
The case Popper attempts to make to rehabilitate Socrates, in comparison, is ludicrousness itself. The near-simultaneous rebellion of nearly all of his students against his earlier philosophy. The evil plot of Plato to co-opt Socrates good name to lend credence to his destructive social ideas by lying about his teacher's true teachings after his death. The participation of many of his students in the bloody oligarchic coup, and the even bloodier purges during their reign that, somehow Socrates opposed (without, however, the citizens discovering his opposition). Plato's own rejection of Socrates' philosophy evidenced by his antidemocratic activities of Plato in Syracuse and elsewhere.
Certainly there were many still alive that would remember and object had Plato given a false impression of Socrates in the dialogs. But since that that very democracy had felt it necessary to get rid of Socrates by death or exile for his role in instigating the now-overthrown oligarchy, it seems likely that (a) their picture of Socrates was not terribly different from the picture that Plato paints of Socrates in Republic and Laws, and (b) having just executed Socrates for crimes against the state, Plato could hardly use Socrates's good name to gain favor for his antidemocratic theories, any more than a modern "philosopher" could use Hitler to lend popular credence to some new fascist social theory.
The conclusion is simple and obvious, and neatly meets Ockham's test. The name behind the evil of Plato's Dialogs was the very man to whom Plato attributes them: Socrates.