Rosetta

Burnham decides to visit a dead planet near the hyperfield which may be the homeworld of Species 10-C. Earth General Diatta Ndoye believes this is a waste of time, but other diplomats support the decision if it provides useful information. Burnham travels to the planet with Saru, Culber, and pilot Keyla Detmer, where they find the large skeletons of a species that once lived in the planet's former gas layers. They also begin to experience hallucinations that create strong feelings such as fear. The team deduce that 10-C fled the planet during meteor strikes that burned away the gas and killed some members of the species, and that the species use complex hydrocarbon compounds to communicate their emotions which are now affecting them on the planet. They believe they can use these hydrocarbons to help communicate with 10-C.

Summary from Wikipedia, CC BY-SA 4.0.