Cheng Xin first wants Halo to return to Earth, but Cao Bin urges Cheng Xin and AA to go to Pluto instead. Cao Bin explains that Pluto is farther from the expanding two-dimensional collapse and may give them slightly more time, though the Solar System will still be destroyed within eight to ten days. Cao Bin also asks Cheng Xin and AA to help rescue artifacts from the Earth Civilization Museum by scattering them in space so their flattened forms may remain intact, and reveals that Luo Ji is alive on Pluto and wants to see Cheng Xin.

Halo’s AI takes control of the voyage once Cheng Xin authorizes the destination. During the trip, Cheng Xin and AA learn that the new Halo is a small but elaborate stellar yacht, designed with redundant ecological systems, direct planetary landing capability, and luxurious miniature natural courtyards. Soon afterward, the Solar System Federation formally announces the dark forest strike: dimensional collapse has begun, all life will be destroyed, laws against Escapism are repealed, but escape is impossible because the collapse can only be outrun at light speed.

After sleeping through most of the voyage, Cheng Xin and AA arrive at Pluto. They land on a huge inscription reading Earth Civilization and exit into the freezing darkness. Following simple wordless arrows, they find a black monolith with hand-operated doors and an old-fashioned air lock, whose low-technology design makes Cheng Xin and AA suspect it was intended to last for vast spans of time, not to suit contemporary humans.

Inside, Cheng Xin and AA meet the nearly two-hundred-year-old Luo Ji, who is frail but mentally sharp and playful. Luo Ji leads them into the underground complex and explains that the Earth Civilization Museum is really humanity’s tombstone, not a museum for visitors. Inspired originally by Secretary General Say’s old dream of leaving a legacy, the project discovered that advanced data storage could not last over geologic time, so the only practical method was ancient and simple: carving words and images into stone.

As they walk through the cavern, Cheng Xin sees the walls covered with reliefs, ancient scripts, poetry, philosophy, and historical records, all meant to survive for one hundred million years. Luo Ji reflects on history’s missed possibilities, while Cheng Xin quietly connects that idea to life itself. The monument’s purpose proves more symbolic than practical, since future beings may never understand the carvings, and now the dimensional strike will destroy the tombstone before it has lasted even a century.

At the artifact halls, Luo Ji tells Cheng Xin and AA to choose what to load onto Halo. AA suggests paintings, and they begin removing frames to save space, including Van Gogh’s Starry Night, which deeply affects Cheng Xin because she remembers seeing it four centuries earlier. Luo Ji asks to keep one painting for himself: the Mona Lisa. Sitting beside it, Luo Ji seems moved by memories beyond the painting, while Mona Lisa’s smile appears eerie and unknowable in humanity’s tomb beneath Pluto.