Here's the plan—Mark will make some H2O by slowly release hydrazine, and separating the hydrogen from the nitrogen. Then, he'll burn the hydrogen, which will slowly combine with the oxygen in the air to create water. It's more dangerous than driving blindfolded, but it's his only shot.
Marks starts the process, storing the water in "a few Hefty-sized bags" and a spacesuit too, because why not (4.42). Everything goes fine for several days—until suddenly, it doesn't.
It's Day 37 and Mark is sitting in a rover, freaking out. See, he had made a few miscalculations that resulted in a ton of extra hydrogen floating around the Hab. It's fair to assume that this doesn't mean anything to you, but allow us to put it in the simplest possible terms–"The Hab is now a bomb" (4.94).