This film is based on a true story about a British teenager who allegedly poisoned family, friends, and co-workers. Graham is highly intelligent, but completely amoral. He becomes interested in science, especially chemistry, and begins to read avidly. Something of a social misfit, he is fascinated by morbid subjects such as poisons and murder. His family environment is intolerable to him and, in particular, his stepmother torments him. He decides to poison those who annoy him, first with antimony and later with thallium. He smugly thinks himself cleverer than all those around him, but nevertheless he is caught and sentenced to 'rehabilitation' at a psychiatric institution. Once there, he undertakes to deceive the new eminent psychiatrist sent there to 'cure' him, thereby securing his release.

Arriving at work one morning, Helen discovers that she had been unjustifiably sacked from her PR job. She is returning home when an amazing thing happens, time reverses itself for a few seconds and a second version of herself is created. In one reality Helen catches the tube train, meets James and arrives home to find her loathsome Lothario lover Gerry cheating on her with his ex-girlfriend Lydia. In the other reality, Helen misses the tube train, gets mugged, goes to hospital and eventually arrives home to find Gerry alone in the shower. The two realities move forward in tandem; in one Helen leaves Gerry and forms a happy, new, loving relationship with James; in the other Helen's live becomes more and more wretched as she takes on two jobs to support her worthless, cheating boyfriend as he supposedly writes his novel but in fact carries on a torrid affair with Lydia.