Deep in the jungle of Central Vietnam, lies a magnificent underground kingdom. Hang Son Doòng, which translates as "mountain river cave," is located in the Phong Nha-Ke Bàng National Park in Quang Bình Province. It is the largest cave passage in the world and a place of spectacular beauty; the national park is also a UNESCO world heritage site. In 2014, Son Doong's future was thrown into doubt when plans were announced to build a cable car into the cave to attract tourism. With many arguing that this would destroy its delicate eco-system and the local community divided over the benefits this development would bring, A Crack in the Mountain follows those caught up in the unfolding events.

Beautifully shot and scored, the film uses this narrative as a lens through which to investigate related themes such as the challenges of modern day exploration, environmental conservation and sustainability, and the perils of operating as an activist in a country such as Vietnam—where freedom of speech is severely curtailed.

The Phong Nha-Ke Bang National Park, in which Hang Son Doòng is located, was recognized as a UNESCO World Heritage site in 2003 in large part due to its otherworldly cave systems. It is widely considered to be one of the best destinations for cave exploration on Earth.