Cheng Xin and Cao Bin travel toward Halo City, a medium space city located with Lightspeed II near the edge of Jupiter's protective shadow. Before they arrive, Cao Bin explains that Lightspeed II was once a research city devoted to lowering the speed of light for the Black Domain Plan, but it has been abandoned. Cheng Xin insists on seeing it despite Cao Bin's warning that the ruined city is considered a ghost city.

The dinghy enters a crack in Lightspeed II's damaged shell. Inside, Cheng Xin finds no buildings or signs of habitation, only an empty frame lit by faint, irregular blue flashes. Cao Bin explains that the light comes from space dust falling into a microscopic black hole at the city's center, a black hole with the mass of the former Jovian moon Leda.

Cao Bin recounts the history of the Black Domain research effort. Scientists spent decades trying to lower the speed of light in vacuum, but no method changed the fundamental constant. The circumsolar particle accelerator finally made it possible to create microscopic black holes, so researchers injected one into Leda, causing the moon to collapse into a stable black hole. Lightspeed II was then built as a giant nonspinning vacuum container around it so scientists could study it under laboratory conditions.

Cao Bin then tells Cheng Xin about Gao Way, the head of the Black Hole Project. Gao Way was severely autistic, socially isolated, and obsessed with changing the speed of light, partly because he identified that impossible change with changing himself. When the black hole still failed to help the project achieve reduced lightspeed, Gao Way became fascinated by the singularity as a place where known natural laws no longer applied.

Gao Way disappeared after entering the protected zone around the black hole, likely by choice rather than accident. Because of relativistic time dilation, distant observers still see Gao Way's figure frozen at the event horizon, while in Gao Way's own frame of reference he has already crossed it. This paradox even prevented his life insurance from being paid, because from the outside the accident will never end.

An unnamed woman who loved Gao Way sued to halt further experiments that might disturb the black hole and endanger him if he remained alive from the outside perspective. Although the lawsuit failed, research stopped and Lightspeed II was abandoned. Cheng Xin, shaken by the thought of Gao Way falling forever in one frame and dead in another, asks Cao Bin to continue on to Halo City.