The Sound of Thunder

Saru, freed from fear, is changing: sharp spines have grown where his ganglia were. When a red signal appears above Kaminar, Discovery heads to the homeworld he left eighteen years earlier. He is reunited with his sister Siranna, who is shaken to find him alive and transformed. Saru confronts the Ba'ul, who rule the Kelpiens from above, and is taken prisoner. Data from the sphere reveals the truth behind his people's fate: Kelpiens who survive the change become formidable, and long ago they nearly drove the Ba'ul to extinction. Ever since, the Ba'ul have culled Kelpiens before the change can happen. Now the Ba'ul move to destroy the Kelpiens altogether. Saru and the crew turn the Ba'ul's own network against them to trigger the change in every Kelpien, and the Red Angel's sudden appearance stops the attack. The two species must now find a new balance, and Burnham sets out for Vulcan in search of Spock.