This one seems to have some dropped words; I’m going to make my best guess as to what they are. Also, note the comma in the second sentence.
The mutants in the forest must not be allowed to advance beyond the tree line. If they do reach our village, we will be doomed like all the others.
I’m not crazy about the use of the word mutants here, partly because I suspect the writer doesn’t actually know what it means, but what can ya do?