When Grenouille first encounters other people, he looks so horrifying that people run from him screaming. When he enters the town of Pierrefort, he's treated as a sensational curiosity and is taken directly to the mayor. Grenouille presents his journeyman's papers and invents a story that he was attacked by robbers and held captive. This isn't so wild of a story, as similar attacks are relatively frequent in the area. The mayor passes his report on to the marquis de La Taillade-Espinasse, a local nobleman.
We see in Grenouille's process that he's using people in a very similar way to how he was once used by people like Grimal and Baldini. His narrative of being a captive allows him to extract the people's curiosity and then kindness, propelling him toward a person who can help him move on and accomplish his goals.
The marquis had left life at Versailles at age 40 and retired to his estate. There he wrote important scientific works on various topics including experimental agriculture. This topic inspired the marquis' research into the relationship between one's proximity to the earth and vital energy, with the belief that living beings seek to distance themselves from the earth to escape poisonous gas concentrated at ground level and below. The marquis, upon hearing about Grenouille's last seven years, finds his theories confirmed by Grenouille's horrific state. He offers to take Grenouille to Montpellier with him to save him from the poisonous gas in exchange for money.
As usual, the marquis sees Grenouille in exactly the same way that Grenouille sees others—as useful. The marquis’ ideas seem patently absurd, but he can seemingly twist any kind of evidence to fit his theory. Notably, the marquis offers Grenouille money in exchange for being a research subject, which will certainly help Grenouille later.
Grenouille and Taillade-Espinasse make the journey in two days. The marquis is excited to present his find to an audience, and when they arrive, sends invitations to local societies to come view Grenouille. A week later, Taillade-Espinasse lectures to a huge crowd on his theory of “fluidum letale” (the supposed poisonous gas of the earth), while Grenouille stands on a dais as a research subject. The marquis asserts that Grenouille is closer to death than life, and the lecture is a success. Grenouille contributes to the success, as he feels perfectly healthy, but pretends he can't speak from exposure to the gas.
It's let on to the reader that this supposed “proof” of that marquis' theory is entirely fabricated—Grenouille is only acting unhealthy. His time in the mountain (which was, furthermore, an “airy” place) was actually a time of great internal growth and discovery for him.
Back at Taillade-Espinasse's manor, Grenouille is locked in a "vital ventilation machine," which is supposed to surround Grenouille with air from higher regions. The marquis feeds him foods from "earth-removed" regions, primarily poultry dishes. Grenouille is treated for five days and then is allowed to bathe, trim his nails, and soften his skin. The marquis calls for a tailor and a cobbler to fit clothes for Grenouille, and the marquis himself applies makeup to Grenouille's face. Finally, the marquis applies violet perfume to Grenouille and steps back to admire his handiwork.
The care Grenouille receives here is reminiscent of the care Baldini lavished on him when Grenouille fell ill. As usual, Grenouille is only treated as a valuable human and cared for when his health and existence are useful for someone else.
Taillade-Espinasse addresses Grenouille as "Monsieur," states with delight that Grenouille now looks like a perfectly normal man, and offers Grenouille a mirror. Grenouille, seeing a gentleman in the mirror, ducks instinctively, but realizes it's his reflection and stands to regard himself. Grenouille is struck by his normalcy, and thinks that except for lacking a human scent, he isn't unusual at all. Grenouille realizes that this isn’t because of the marquis' “treatment”; his appearance of normalcy is purely thanks to the clothes and the makeup. He thinks that this figure in the mirror could have an effect on the world.
While it's obvious that the marquis' theory has nothing to it, what's important here is that Grenouille learns that he can pass for normal, save for his lack of scent. By donning the clothes and the makeup of a gentleman, Grenouille understands that he can now exist in society without raising questions as his appearance once did. Further, he realizes that the person in the mirror can have an effect on the world, which sets him up to cultivate a personality and persona.