The Passionate Sentry (1952)

"Five-Sided Triangle", while geometrically incorrect, might be a better title for this film that the two it has. A private in the Guards is forced to leave his girl friend in the (not-for-long) empty house of a high-ranking civil servant. The son discovers her and promptly falls in love. When the daughter returns, she decides that it would be best if the girl got together with her boy friend, not realizing that the sentry has worshipped her from afar, the daughter. A further complication is that the sentry's commanding officer is in love with her also.