Rubashov is an old bolshie, a hero among the revolutionaries, and a loyal party enforcer.  But as his idealistic generation is almost entirely purged, he becomes disillusioned with Stalin, and makes some sarcastic comments to the wrong comrades.  He learns that the new party has no sense of irony, and finds he has an expensive debt to pay them.

That is the story line, but the real subject of the book is the duality of the mind; the capacity for rationalization and self deception--ultimately, the nature of identity.  Also, that perhaps there are some means that do not justify the ends.

Finally, it is a dramatic masterpiece, stitched together with Rubashov's interior monologue (which Koestler turns into a Buber type dialogue), his written statements (including a new historical dialectic), his conversations with prison inmates conducted by coded tapping on pipes, and most importantly, his interrogation and trial.

All that packed into 200 pages, each one of which sizzles.