Maybe the most important thing about Secret People is the fact that William Wyler took a look at this film and decided that his next film Roman Holiday would star an unknown Audrey Hepburn. Hepburn plays a supporting role as the younger sister of Valentina Cortesa. Both are refugees from some unknown eastern European country where the two of them had their father killed by the local dictator.
Audrey was still a kid when she and Valentina came over, but now she's grown up and an aspiring dancer. As for Cortesa she's content enough until Serge Reggiani shows from the old country. He's with the opposition to the dictator and they want to kill him in London while he's on a state visit. So far it sounds like the plot of Hitchcock's The Man Who Knew Too Much.
But this film is told from the point of view of the conspirators and how slowly Cortesa is drawn into their web of intrigue despite a lot of misgivings. Every agonizing thought so registers with Cortesa and her performance even after Hepburn who has adjusted well to Great Britain and wants to pursue a career in dance.
As for Reggiani the years have turned him into quite the fanatic. Today he would be called a terrorist.
Secret People is done a bit unevenly in pace, there are spots it drags. But Cortesa and Reggiani carry it through and it's a milestone of sorts for Audrey Hepburn.