Mattos summary

Moshe teaches the laws of vows and oaths to the Children of Israel. The father of a young girl may nullify the vow of his daughter immediately upon hearing it, as can a woman's husband. If they do not nullify it, it becomes binding. No person shall break their word or oath.
G-d instructs Moshe to wage war against the Midianites, after which he will die. Pinchas leads a thousand warriors from each tribe into battle and they kill all the men of Midian. However they take all the women and children captive. Upon their return Moshe chastises them for sparing the women, who were the one's responsible for seducing them to idol worship which led to the plague amongst the Jews. He instructs that all the boys and all women old enough for intercourse be put to death.
The Torah explains the laws of koshering utensils in regard to the spoils that were taken from the Midianites. Any vessel that is used in cooking must be koshered by heating. Also, any cooking utensil made by a non-Jew must be ritually immersed in a Mikva before being used.
The spoils of battle are divided up, half to those who fought and half to those who remained and guarded the camp. A tithe is also given to the Levi'im. The commanders of the soldiers bring a thanksgiving offering in recognition of the fact that not a single Jewish soldier was lost.
The tribes of Reuven and Gad see that the land on this side of the Jordan River is ideal for livestock. As they have many animals, they request from Moshe that they be allowed to stay here instead of entering into the Land of Israel proper. Moshe at first criticises them for preferring to remain in the desert, but then allows them to stay there provided that their men first go into Israel with the other tribes and help them to conquer it. The tribes of Reuven and Gad agree to this condition, and they and half of the tribe of Menashe remain on that side of the Jordan, building cities for themselves there.