Phillipe, the son of an ambassador in London, idolizes Baines, his father's butler, a kind of hero in the eyes of the child, whose perception changes when he accidentally discovers the secret that Baines keeps and witnesses the consequences that adults' lies can cause.
Some lies are just kindness.
I know your daddy!
The real star of The Fallen Idol is the embassy itself: its marble checkerboard receiving parlor, its outsized catenary staircase. From the film’s opening, when Henrey stares poignantly through the newel posts at his father’s departing entourage, to the final bit of detective work at the end, it is the physical space of the film that ultimately remains. For all its other virtues, The Fallen Idol leaves us with a dark, splendid moral ambiguity—and with that set of stairs, massive, curvilinear, indelible. It is a staircase as poignant as the one in Letter from an Unknown Woman, as full of drama as the one in The Magnificent Ambersons, as riveting as the one in Suspicion. And, perhaps, as deadly as the one in Psycho.
— Howard A. Rodman (Criterion)
Like The Third Man, The Fallen Idol is a postwar movie in which war is shown to have no end: the battle continues in parlor and kitchen. […] [Phile] is, of course, not just any child, but the privileged son of a diplomat, inhabiting an embassy of palatial intricacy. We sense the privilege in his physical delicacy and in an arrogance that can be forgiven only because it is utterly unselfconscious. Privilege here quickly becomes indistinguishable from loneliness and silent suffering. […] In the solipsistic world of childhood, space exists in order for Phile to play in it, and adults exist to assist him in his play, giving him boxes for his pet snake or taking him to the zoo (to see the snakes, of course). If he stares out into their world, it is initially as if it were a spectacle provided for his enjoyment. But the longer and harder he stares, the more his situation becomes that of the child who has wandered by mistake into a movie for grown-up.
— Geoffrey O'Brien (Criterion)
Phile […] keeps a small pet snake behind a loose brick on a second-floor balcony, but there are larger snakes about, morally and metaphorically. […] 'The Fallen Idol is cruel in the way it confronts Phil's small mind with the endless mysteries of adult responsibility — but it hints that a child's naiveté can be a lethal weapon on its own.
— Ty Burr (Boston Globe)
As the film's putatively happy (and strange) windup makes it clear, when childhood ends, there really is no place left to run.
— Manohla Dargis (The New York Times)