In their base over the cold winter months, they have a strong community spirit and more join their number from neighboring ghettos. Tuvia reclaims his leadership by shooting dead an insurgent, organized patrols bring in food, and they even have time for classes.
When Spring approaches, Zus (who is still with the Red Army) learns that the Germans are planning a full scale attack on the forest - but that his commander is going to leave the Jews to their death. If he doesn't leave with them, he will be shot.
The youngest brother somehow captures a huge German soldier, and from his papers the Jews in the forest learn that the attack is imminent. Tuvia for the first time begins to doubt the people in his care when some of them attack the German like a pack of animals -- something he has always tried to avoid.
A spy plane flies overhead - followed by bombers that destroy their forest camp and scatter the people. They quickly make their way out as the infantry will soon be approaching. Those left behind to defend their escape don't last very long.
Tuvia leads the people out the forest to a great lake. Taking his cue from Moses, they hold hands and make their way out through the strong currents.
They make it safely to the other side (all except the old school teacher who dies of fatigue), but can't relax for long as a Panzer division approaches and opens fire. Tuzia and the academic outflank the platoon and steal a machine gun, but are trapped and the academic kills himself as he can't throw the grenade very far.
Suddenly, Zus leads his men out behind the Germans and with their better military expertise kill the remaining troops, and throw a grenade inside the tank. Zus asks to rejoin his brother's mission, and he gladly accepts.
As it ends, they enter another forest and the credits tell us that 1200 survived two more years of hiding - and in the new camp they even opened a school and a hospital. Later, Tuvia and Zus moved to New York where they started a trucking company, the middle-younger brother (Jamie Bell) joined the Red Army but died 6 months later. The number of children and grandchildren from those that survived now numbers in the tens of thousands, but the Bielskis have never sought credit. (The other side to the story and possible reason is that Tuvia's men were just as bad as the Germans, but that wouldn't have made as good a movie)