Pierre has got soul, but he's not a soldier. Never mind. The Count is still ready for some action when he wakes up to discover that the Battle of Borodino is in full, bloody swing. "I should like to get in the thick of it," he announces to General Kutuzov, who rolls his milky eye in response. In true trustafarian fashion, "getting in the thick of it" means blocking the real soldiers' way and chuckling to himself. Pity he didn't bring a selfie stick.
Then shit gets real, and Pierre barely avoids getting split in half by a cannon. Andrei's not so lucky. When a cannon rolls toward his band of soldiers, he ignores calls to get down and instead opts to stare the bomb down. Ka-boom! He's on a gurney with a serious chest wound while a sobbing Anatole has his leg chopped off right beside him. Andrei somehow manages to not get up to chop off the other leg, instead reaching out a hand to console his former rival in love.
The Russians have lost half their army and plan to retreat. As Petya warns his fellow Rostovs, the French are coming to invade Moscow. Natasha shames her mother into making room for some wounded soldiers as they flee the city. On the way out, she spies Pierre in ragged peasant's clothes. He refuses to join her on the grounds that he needs to stay in Moscow to kill Napoleon. Oh, Pierre. You do say the funniest things.
Instead of killing Napoleon, Pierre heads home, where he finds a boozy French soldier making the place his own. The two dine and swap love stories like old comrades. When the soldier falls asleep, Pierre takes a knife and heads out. The French have torched Moscow, and the clumsy Count finally gets his moment of glory by saving a baby from a burning building. Drunk on power, he fights some French soldiers and winds up getting arrested. Womp.
En route to the countryside, the Rostovs stop to help more wounded soldiers. Sonya/Anna Kendrick discovers that Andrei is among them. As soon as she learns that her former fiancé is seriously wounded and living under her own roof, Natasha makes a beeline for his bedside. There, she begs for forgiveness, but Andrei's not having it. It's him who should be forgiven, he says. Either his injuries wiped out all memory of that hot fling with Anatole, or he's had another battlefield epiphany. Either way, he and Natasha are back in love.
The Countess, meanwhile, is determined to sacrifice Sonya/Anna Kendrick to the love gods. With the family's finances down the drain, she makes it clear that Nikolai needs to marry up (i.e. Princess Marya) and not down (i.e. Sonya/Anna Kendrick). The latter writes Nikolai a letter to free him from their engagement, then (we assume) goes to her room to stab her Marya voodoo doll.
Speaking of Marya, Marya, Marya, she rushes to Andrei's bedside. The prognosis is grim, and Andrei doesn't sugarcoat things when he gives his son the idol he wore into battle. He says his goodbyes, the last rites are said, and he slips into a dream state in which he sees images of Lise, his first wife, and his beloved Natasha. We're not going to cry. We're not going to cry. We're not going to cry.
Part Two begins with Pierre in prison, having been spared the firing squad. He makes friends with a philosophical fellow prisoner, who teaches him to savor his cold scrap of potato bit by bit. Pierre's never met a self-help guru he hasn't liked, so it's no surprise to see him welcoming this mindful new lifestyle.
Meanwhile, his wife Helene is in a different sort of prison: her own pregnancy. Pierre hasn't responded to her request for an annulment, and her blossoming body is making her indiscretions public. After being snubbed by St. Petersburg society, she resorts to knocking back all of the miscarriage-triggering potion an Italian consultant has given her. Lo and behold, it kills her.
The Russian winter is scaring off the French. Napoleon orders his troops to march out, and they round up Pierre and the other prisoners to join them. Before long, the Count has gone full Revenant, with an icy beard and little will to live. His philosophical friend has been killed, and things are looking bleak.
As the French troops retreat, Denisov and Dolokhov are leading a band of Russian soldiers to pick off their enemies. Their latest recruit is little Petya Rostov, who looks like he'd faint if the wind blew. Denisov vows to keep him safe, which can only mean one thing: Petya is as good as dead. He manages to survive a mission into enemy territory with Dolokhov under the guise of being French. The next morning's sneak attack on the French, however, leaves him dead, prompting howls from Denisov. On a happier note, Dolokhov has discovered Pierre among the prisoners. Apparently that whole shagging-Helene-on-the-dining-room-table incident is all vodka under the bridge.
News of Petya's death hits the Rostovs hard, and is soon followed by the death of their patriarch, Count Ilya. He's buried in Moscow, where the family have been reduced to poverty. When Marya offers to house them, Natasha and the Countess gratefully accept. Nikolai, fearful of being branded a golddigger, digs his heels. Once Marya convinces him that his change in financial status is of no concern to her, he proposes marriage.
After a fresh shave and a more mindful approach to eating potatoes, Pierre pays Marya a visit. He dazzles her and Natasha with tales of his plight, but has a more pressing issue for his next visit. Could Natasha ever love him? Despite her bond with Andrei, Natasha doesn't miss a beat. She not only could, she does. And Duckie finally gets the girl.
Flash-forward to a happy montage some years later. Andrei's son is a strapping teenager. Natasha and Pierre are blissfully married with children, as are Nikolai and Marya. Even the Countess has managed to crack a rare smile. Oh, and there's poor, single Sonya/Anna Kendrick acting like it's all fine. The end.