I (Mark Watney) and my team have reached Mars to collect some samples and know about Mar. We work for NASA.
Due to sandstorm everyone left except for Mark Watney because everyone in his crew thought he dead because an antenna hit him.
I found himself in the sand storm. I got up and found myself wounded I somehow manged to get back to Hab.
I also found enough food enough for myself to survive for some days but would be needing more while I was searching for more food I found potatoes.
Then an awesome idea came decided to grow some potatoes inside the hab. I used my and my crew member waste to grow more potatoes.
When I was growing potatoes I realized that I didn't have enough water for the plants so I decided to make some more water. Mark Watney used that same reaction to produce the hydrogen gas and then, in combination with the oxygen in the hab, I burned the hydrogen and made water. I bet I am way more smart then I thought.