A martial arts instructor yesterday denied plotting with two teenagers to frame a mentally retarded man for indecent assault to seek revenge against him. Shum Yat-sing, 33, held a grudge against Due Wing-kwai, 18, who had accused him of being a triad member and demanding protection money, prosecutor Allan To told the District Court. Mr To said Shum, who is also a bus driver, conspired with two martial arts students - Li Yuen-ting, 16, and a 15-year-old who cannot be named - to pervert the course of justice. Shum and Li pleaded not guilty to the charge but the 15-year-old pleaded guilty. Deputy Judge Henry Mierczak heard that on November 9 last year, Shum phoned the teenagers and asked them to set Mr Due up by complaining that he indecently assaulted Li. That night, Li complained to Shum, who was on duty with KMB, that she had been assaulted by Mr Due, who was later arrested. Mr Due was acquitted by Tuen Mun Court in February. Deputy Judge Mierczak transferred the 15-year-old's case to the juvenile court because he was 14 at the time of the offence. The trial of Shum and Li continues.