They start off by introducing the runners the documentary will focus on which includes a marine, a man with a prosthetic leg, a 68 year old man, and the current course record holder. They also showed some of the training they do to prepare and it includes running in a sauna for hours on end as well as running on a treadmill, layered in clothes, with a dryer hose blowing heat on them. Many people who run this are just trying to become an "official" finisher, someone who completes the 135 mi in under 60 hours while others are trying to earn the cherished prize, the Badwater belt buckle, by finishing under 48 hours.
The documentary follows these runners (and their training crew helping them on their journey) from the enthusiastic first 15-20 miles, to the painful miles in the middle of the night, to the tears of joy at the finish line. Some of the racers go through agonizing pain during the race with dehydration (just imagine running in 125 degree weather for hours), stomach issues, as well as severe blisters on their feet and toes. Trust me, some of it can be graphic but incredible what they overcome just to finish. At some points, racers are literally laying on the side of the road in so much pain you don't think they can go any further, but they pull through.
If you think running a marathon is crazy, then these people are absolutely insane. These people are running more than 5 marathons in a row! It was amazing to see how strong the human body and mind really is. Because it was such an inspiring film for runners & non-runners, I would definitely recommend watching it.