Naruto and Sasuke are flung away from Kaguya. Sasuke is plummeting towards the lava, but uses shunshin to escape (although he does lose his sword), and then stops Naruto from spinning out of control. The two of them regroup, but Kaguya emerges from some sort of tear in reality behind them.
Amazingly, she's crying.
Black Zetsu starts to drain Naruto and Sasuke's chakra and explains that Kaguya is crying because they remind her of Hagoromo and Hamura. He then reveals that he, too, is a child of Kaguya, and that he has manipulated the history of the shinobi world to make Kaguya's return possible. The Juubi was apparently a composite of the Shinju and Kaguya, so when Hagoromo and Hamura sealed it, they sealed Kaguya too. Shortly before this, Black Zetsu was born. He exerted influence over Indra after he was passed over for Hagoromo's inheritance, and continued to manipulate the Uchiha by altering the writing on the stone tablet in the Nakano Shrine to say that Mugen Tsukiyomi would save the Uchiha.
Each time Indra and Ashura reincarnated, Black Zetsu approached them, trying to set up a situation where one of them would awaken the Rinnegan, but this always failed...until Madara and Hashirama came along. During their fight at the Valley of the End, Madara bit off a piece of Hashirama's flesh. Tobirama hid Madara's body, intending to study the Sharingan, but unbeknownst to him, Madara had set an Izanagi to activate, resurrecting himself.
Madara then regurgitated the piece of Hashirama's flesh and implanted it into his own wounds. This allowed him to awaken the Rinnegan and summon the Gedo Mazo. Black Zetsu, meanwhile, started Akatsuki and used Madara's corpse as leverage over Kabuto. Finally, his plans came to fruition: Madara activated Mugen Tsukiyomi, and with the chakra it was draining from the people trapped inside it, he was able to unseal Kaguya.
Naruto retorts that Black Zetsu wasn't the only one to create the history of shinobi and reprimands Kaguya for not being proud of her sons for becoming independent of her.
He rips Black Zetsu off himself and Sasuke, and the two conclude that they must use "that jutsu" to create an opening to seal Kaguya.
This chapter was something of an infodump, but an interesting one. The questions of how Madara survived the VOTE battle and awakened the Rinnegan were answered, and we learned about the ultimate roots of the animosity between the Senju and Uchiha as well. We've heard the Uchihas' "curse of hatred" referred to several times, and I wonder how much of this supposed curse was actually Black Zetsu's influence. How might things have been different if Black Zetsu hadn't altered the writing on the tablet? Madara and Itachi both obviously knew about it, and Madara's desire to activate the Mugen Tsukiyomi was at least partly based on the false writing. How might Itachi's (and Sasuke's, since he read the tablet too) decisions have been affected by the alterations Black Zetsu made?
Black Zetsu being Kaguya's child was also interesting. (Was anyone else reminded of Melisandre birthing the shadow creatures on Game of Thrones? Reading this chapter, I got a mental image of the two of them sitting together, knitting little booties for their baby abominations.)
Madara's obsession with Hashirama continues to get even creepier. Some people in fandom have commented on Naruto relentlessly pursuing Sasuke whether Sasuke actually wants to be friends with him or not, but at least Naruto hasn't cannibalized Sasuke yet.