Thirteen year old Michael heard a couple having a blazing row down by the canal when he took a shortcut home. The next day he hears that a woman was killed there, and he realises that he had actually seen the murderer just before he struck. But he can't tell anybody! It would mean admitting that he had been somewhere that he really wasn't allowed to go. So he turns amateur detective, befriends the victim's young cousin, and scouts out new evidence. As the police concentrate on investigating the family, they will never find out about the true murderer unless Michael tells them about him. And Michael is being chased down to the canal by the murderer himself, before he has the courage to ring the police.