Frankie (Andrew Ray) is hanging out the window of his parents' flat, eating an apple and amusing himself watching a street escapologist being moved on by a policeman when he notices a boy of around his age walking past with a yellow balloon. He is immediately envious, and asks his mother Em (Kathleen Ryan) if he can have one too, but she dashes his hopes by refusing. There is still hope, as his father Ted (Kenneth More) returns home and gives him enough money to buy the plaything, but alas in Frankie's rush he trips and the coin rolls down a drain. His thoughts start to scheming: there's a boy he knows, perhaps he can persuade him to hand over his balloon...
Writers J. Lee Thompson (directing his second film) and Anne Burnaby were evidently most impressed by the memorable American thriller of the forties The Window when it came to scripting The Yellow Balloon. They may start from different points, but they end up in the same place, as the tenements of one city are replaced with those of another, although London had bombed out buildings that children were warned against playing in, which is precisely where Frankie is chased to by his friend once he steals his balloon. You can see where this is heading, and before long the other boy has fallen to his death.
Frankie is mortified, not least because it is partly his fault they were running around in a dangerous ruin in the first place, but it quickly gets worse for him when it transpires somebody was watching: a local ne'erdowell called Len (William Sylvester). This man is such a scoundrel he threatens to go to the police if Frankie does not do as he says, yet he couches this in "poor me" terms designed to make the boy feel sorry for him. What Len wants is money, and to get the boy to steal for him, so the plot becomes a battle of wills between the decent upbringing of his parents and Len's manipulation.
What this does have in its favour is Gilbert Taylor's excellent cinematography, which brings the actual locations to life in vivid black and white, full of deep shadows to mirror the lead character's descent into a hell of guilt and forced law-breaking. Even if the plot fails to engage, the moody visuals do compensate as while we are safe in the knowledge that Frankie will be saved eventually, the look of the film tells a different story where his world has transformed into a menacing maze of accusing looks and grown-ups who would be sympathetic as long as they don't know the reason you're carrying such a weight on your shoulders.
It will get worse for Frankie before it gets better as Len persudes him to steal for him with the sob story that he needs the money desperately and the boy is the only one who can help. Frankie responds by trailing around the city at night (of course) and watching for opportunities to nick some cash, although what he does end up stealing is a pineapple from the fruit barrow of Sid James (you knew he was going to turn up somewhere in this, didn't you?). Len doesn't have much use for the fruit, but he has an idea that will place the child in danger when he is used as a way into a pub after closing time one night so that Len may rob it. Needless to say, this ends badly, and to be fair the climax which sees a murderous Len tracking Frankie through the tunnels of the London Underground is respectably suspenseful. It's the sinister imagery of The Yellow Balloon that makes it worthwhile, though. Music by Philip Green.