This documentary follows a group of slack liners walking their lines. The film shows harmless slack lining above the ground as well as advanced slack lining across the crevasses of Mont Blanc and the legendary Lost Arrow Spire in Yosemite National Park (USA). Its impressive pictures provide an insight into what this sport means to a slack liner. The viewer witnesses the athletes’ emotional ups and downs and shares their personal borderline experiences. The slack liners also tell us about the origin of the sport and the know-how and technical knowledge necessary to safely put up a high line.