Popper mounts many-faceted defence of liberal democracy and its underpinnings against what he sees as attacks on it by Plato/Aristotle (essentialist, authoritarian) and Marx/Engels (historicist). These great tidy, totalising theories of society stand against the messy muddling through of democracy.
I was largely in agreement with Popper through much of the text. I'm sure I would re-read more critically now (I am an anarchist), but remain grateful to it for furnishing me with criticisms of essentialism, tyrannical positivism, and historicism, the dubious belief that a certain outcome (revolution) is inevitable, foretold by a discernable pattern of historical events.
Popper doesn't only critique Plato & Marx; he also expounds his own (liberal as in John Stuart Mill) position on socio-political philosophy and the whole book is very wide ranging. The foundations of Popper's well known philosophy of science (falsificationism) and his theory of reason as faith are key building blocks. Fellow opponents of head-in-the-sand rationalism might find Popper's demolition of a priori justifications for reason handy.