Burnham leads a team to investigate a planet which was once inhabited by Species 10-C, and Book and Tarka infiltrate Discovery.
Tropes:
- Call-Back: A Lumerian is sitting in the ship's bar nursing a drink, just sitting there in silence. Why does that look so familiar?
- Dyson Sphere: The star near the gas giant has several Dyson rings surrounding it, noted to be similar in composition to the DMA controller. Whatever purpose they served, they've long been abandoned.
- Hero Worship: It turns out that Adira admires Detmer, but doesn't know the best way to express it. Reno advises them to befriend them further, and not use the words "I want to be just like you."
- Homeworld Evacuation: 10-C used to live in a gas giant just outside the hyper field, but were forced to abandon it when it came under threat by meteor storm. Not all of them could be evacuated, however, leaving behind a mass grave of those forced to watch the end of their world. In the present most of the atmosphere has been burned away.
- No Biochemical Barriers: 10-C's emotion hydrocarbons affect Kelpien and human biology exactly the same, which is lampshaded as rather bizarre.
- Offscreen Moment of Awesome: Tarka is caught in engineering by Reno and the scene ends. Between then and Book's return to his ship, Tarka somehow subdued and kidnapped Reno without alerting anyone on Discovery.
- Starfish Aliens: 10-C are, based on the available evidence, a species of insect-like Living Gasbags that dwell in the atmosphere of gas giants. They communicate through complex hydrocarbons that can convey emotion and even memory, the residue of which remains effective a millennium after they've died.
- Supernatural Fear Inducer: Saru, Culber, and Burnham (in that order) are affected by the fear residue of the deceased 10-C on the gas giant, seeing visions of the death of their world and the fear they felt as it happened.
- Title Drop: The episode is named after the Rosetta Stone, which several characters make references to. In this case, instead of a stone slab, they're dealing with hydrocarbons that impart emotions. It's lampshaded that the stone actually has three languages, not two.
- We Need a Distraction: Tarka sabotages Discovery's replicator system so the staff in engineering will leave to repair them, giving him the time needed to disable the proximity sensors so Book's ship can latch onto the hull undetected.
