Coming down is the hard part, most mountain climbers will tell you, a maxim proved to tragic effect in August 2008, when 11 of 25 experienced adventurers from five different countries perished while descending from the top of K2, the mighty Himalayan peak. The 11 died, one by one, in a series of mishaps that defy climbing logic. In this enthralling if slightly uneven documentary, director Nick Ryan and screenwriter Mark Monroe (The Cove) use survivor testimony, footage shot by the climbers themselves, and seamlessly integrated reenactments to create a plausible scenario for what might have happened. The Summit is at its most powerful when the filmmakers simply tell the tale, which gradually develops the unsettling suspense of a horror movie, with K2 cast as the implacable killer. Many bodies are left on the mountain, but when the living reach the base, the filmmakers soldier on, interviewing survivors and their loved ones in an attempt to separate fact from myth. It’s a noble effort, but by wading into the murky waters of who acted heroically, and who, perhaps, did not, Ryan loses touch with the mountain itself, which took 11 lives on a “perfect day,” and whose majesty, despite that body count, remains an irresistible draw.