A 13-year-old boy is found kicked to death at Euston Station. DS Brooks and DS Devlin discover that the boy's mother is a reformed drug-addict whose boyfriend is suspected of physically abusing her son, and are concerned about events at the boy's foster home. When the case comes to trial, the accused is represented by James Steel's ex-girlfriend, who advances the novel line of defence that the accused was genetically predisposed towards murder.