The donor party leaves on their journey to get to California. They had heavy wagons and had to throw items out to get there faster.
The donor party tried and cut off 100 miles of there journey, so they take a shortcut but get caught in a storm and a serious of mishaps and mistakes take place and are snowbound without the correct supplies.
The last survivors of the donner party after being stranded had no choice but to eat each other. But there was one rule, you couldn't eat your own family.