A teenage boy is found badly beaten, and a young girl, Lisa, from the same school is missing. DC Webb shows little interest in the case, so Sgt Cryer investigates the girl's disappearance himself. A local 'photographer', Andrew Morton, is the main suspect, as he has an obsession with Lisa and her friends, and he is arrested when PC Harker spots him acting suspiciously by the canal. When Lisa turns out to have been hiding out at a friend's house the whole time, Cryer must rush to prevent Lisa's father from taking the law into his own hands by killing the innocent Morton.