"Das Versprechen - Erste Liebe lebenslänglich" is a documentary about Jens Söring who was sentenced to a lifetime in a US prison for the double murder of his girlfriend's parents in 1990. To this day, Söring claims his innocence. In March 1985, the then 19-year-old son of a German diplomat was accused of having murdered Elizabeth Haysom's parents in Virginia. Söring and Haysom fled to England, where they were arrested in 1986. Haysom accused Söring of being the murderer of her parents and confessed to have instigated the murder. He was handed over to US authorities in 1990 and was found guilty despite asserting his innocence and dubious court proceedings. Journalist Karin Steinberger and filmmaker Marcus Vetter try to reconstruct the progression of events before and after the murder. They interviewed Söring's lawyers and accompanied a detective on a search for new clues. The documentary features further interviews with witnesses, Söring's parents, police officers and lawyers who are partly in doubt of his guilt. The film also has Söring tell his version of the events that took place.