George Hamilton is effectively cast against type as a traitor working for the North Koreans to steal important missile secrets back in the USA after refusing a decade before the offer to return to America as a freed POW from a Korean prison. His pretty-boy good looks and breezy manner makes for a far more interesting protagonist than just a stereotypical "turncoat".
He's been training for the mission and is an expert at fooling everyone, but gradually we see that behind the facade of a calculating spy beats the heart and sensitivity of an empathetic human being. Casting grown-up Margaret O'Brien as his potential romantic interest was also a wise decision, as she instantly generates both pathos and because of her child stardom a certain amount of suspicion: could she be as big a fake as George, out to trap him for the good old USA and quash his mission? Completing the fine casting is Carroll O'Connor as a true villain (before Archie Bunker he was terrific in baddie roles) playing George's contact and controller and Jack Weston as his comedy relief new boss.
Bert Leonard's oppressive voice-over narration hurts the show - he doesn't get a credit but that voice is instantly recognizable after recently watching dozens of episodes he narrated for his show "The Naked City" on Tubi.