Exhausted, Ryland Grace returns to the tunnel and finds Rocky has built a small xenonite transfer chamber—a shared mini-airlock—allowing safe exchange between atmospheres. Grace tests it with a tape measure; the rubber grip melts in the hot Eridian air, but the metal survives. Rocky treats the tape as a toy, and Grace’s attempt to demonstrate centimeters fails.

Grace realizes Rocky cannot read ink and deduces that Eridians perceive their world via sound, not light. Clues include Rocky needing the human clock pressed against the wall and being unable to detect thin markings. Grace proves it by holding up fingers behind an opaque aluminum panel; Rocky still mimics the numbers through sound conduction.

Grace infers Eridians use technology to perceive space where no sound travels, marvels at their achievements, and refocuses on communication. He plans to use audio tools to analyze Rocky’s language rather than visual cues.

In a flashback, Stratt appears alone in U.S. federal court as an IP consortium sues Project Hail Mary for wholesale copying of software and literature and alleged patent violations. She produces treaty immunity and a preemptive presidential pardon, effectively ending the challenge, while military personnel underscore her authority.

Back in the present, Grace installs waveform-analysis software with Fourier transform capability and sets up an Excel sheet for Eridian–human time conversion. Rocky doesn’t return immediately, so Grace calculates and posts a stick-glyph time for his return, mounts a small camera to monitor the tunnel, and finally secures himself in the bunk to sleep.