A 13-year-old boy is found kicked to death at Euston station. Mandy, his mother, is a former drug addict and her boyfriend is in the frame as the murderer because he is already suspected of physical abuse towards the victim. At the trial, defence barrister Beatrice McArdle advances a bizarre line of defence - her client is "genetically predisposed" towards murder and, therefore, has no control over their actions.