Quick recap
Now captain, Burnham watches a giant gravitational anomaly, the DMA, appear out of nowhere and destroy Kwejian, Book's homeworld, along with his family. The anomaly vanishes and reappears elsewhere, and nobody knows where it comes from. Scientist Ruon Tarka joins the investigation but has his own plan. Book teams up with him to destroy the DMA by force, against the Federation's position. The anomaly is the work of Species 10-C, a civilization so different it doesn't even realize it's killing people. Burnham leads first contact, a hard conversation held through chemistry and math. The 10-C shut down the DMA, Tarka is gone, Book is forgiven, and Earth rejoins the Federation.
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