Two children get extremely excited about a circus arriving in the railway sidings opposite their homes in its own custom-built train. Unfortunately they are not the only ones interested in this event, as a gang of thieves are planning to use the circus, and one of its performers, as cover for a major robbery. The children, and three more of their friends from primary school, several times come face to face with the robbers. In one encounter one of the friends comes within seconds of losing his life.

At their final meeting before moving to a senior school, they discover that it is the robber's intention to commit a robbery in the centre of town. First they think it is going to be a bank robbery but then realise it is jewels the thieves are after. Through a daring plan it is actually the children who end up with the proceeds of this theft. The villains are not happy about this and, just as the children are trying to hand the jewels over to the police, they accost them. Will the children be able to escape and help capture the culprits?