The film manages to be more captivating than one might expect, featuring John Gregson in the role of a detective, Insp. Parnell, who is tasked with looking into the abduction of Chester’s (Alec Clunes) son. What's notable is that the kidnapper, Marlow (Robert Shaw), has secured the boy within an attic room at a secret London location and then traveled to the father’s residence to request the ransom payment face-to-face—meaning all of the characters are introduced from the beginning. About halfway through, a confrontation occurs, and it turns out that the officer must locate the child on his own. The unfolding events create considerable tension, and the ensemble cast propels the narrative forward effectively while also generating a palpable sense of danger.

Source on TMDB