In 2013, after Numbenchapol's debut documentary feature Boundary (ฟ้าต่ำแผ่นดินสูง) that examines the border dispute between Thailand and Cambodia was premiered at 63rd Berlin International Film Festival, it had been banned by Thai ministry of culture as "a treat to national security and international relations"[1] despite the fact that rating system has been enforced under the Motion Pictures and Video Act B.E. 2551 (2008).[2] However, the committee later reversed their decision on condition that some dialogue must be muted.[3] Boundary was later rated 18+ and was self-distributed by the director at limited cinemas in Chiang Mai, Khon Kaen, Udon Thani and Bangkok respectively, from June 27 – July 21, 2013.[4]
Boundary was inspired in part by Numbenchapol's desire to learn the truth behind the 2010 Thai political protests.[5] The film was screened at numerous festivals such as Milano Film Festival in Italy, Yamagata International Documentary Film Festival in Japan, and Human Right Dignity International Film Festival in Myanmar. Numbenchapol also received Young Filmmaker award from 22nd Bangkok Critics Assembly from this feature.
Numbenchapol's second documentary feature By the River (สายน้ำติดเชื้อ) premiered at the 66th Locarno International Film Festival and received Special Mention Award in August 2013. The film deals with another controversial issue in Thailand: the 15-year-suffering of Karen villagers from the lead contaminated river at Lower Klity in Kanchanaburi province.[6]
By the River received praises from many renowned events, for example, Hong Kong Asian Film Festival, Festival dei popoli in Italy and World Film Festival of Bangkok in Thailand, and was officially released in Thailand on May 8, 2014.