Adventure film involving two boys who wage a battle against diamond smugglers with radio controlled model Spitfire aeroplanes loaded with darts.When their model plane crashes Mike and Harry are befriended by ex-pilot Charlie Bradford and his niece Maggie, who teach them how to fly radio-controlled planes. When crooks steal Charlie's latest model in an attempt to smuggle a stolen diamond across the Channel, the children embark on an ingenious and exciting plan to thwart them.