Although the characters of Keith and Stephen annoy you because they are not only immature but also slightly dense the idea that they have stumbled onto something does start to stir the story.
The character that steps forward increasingly is Keith’s mother herself. The alleged German spy appeals to Stephen directly to stop following her and to make her life easier after they rumble that she is leaving things in a box for someone.
The box lies through the railway tunnel at the right at the entrance to the drive and the boys still think it is a German spy. But by now the penny is starting to drop that this is a relationship with a man and the boys get closer to not only catching the mother but also to discovering the identify of the mystery man.
The relationship between the narrator Stephen and the mother of his best friend starts to become central and that is the fault he seems to have returned to face up to.