The Butler, Proposing is Chapter 156 of the Kuroshitsuji manga.
While the other maids are lowering and trying to disassemble the chandeliers in the main hall, Mey-Rin worries about Ran-Mao. Throughout the day, she tries to get away to search for Ran-Mao, but her maid duties keep her busy. When Mey-Rin finally finds an opportunity to sneak away, Jane catches her and tells her to get back to work.
At night, Mey-Rin gears up to search for Ran-Mao and lowers herself down on a rope from her bedroom window to get to Chris Heathfield's room without being seen by Jane who wanders through the corridors. When she enters Heathfield's bedroom, there is nobody inside, but she notices a draught coming from somewhere although the windows are all closed. Mey-Rin follows the trail of the draught and pushes away a bookshelf to reveal a hidden staircase.
Mey-Rin climbs down the stairs which lead to a room filled with hospital beds and machinery. The beds are filled with all the maids Heathfield summoned to his bedroom and liked. They are all fast asleep and connected to the machines which carefully draw their blood without killing them.
Right after Mey-Rin finds Ran-Mao in one of these beds, she is attacked by Jane. Holding each other at gunpoint, Jane tells Mey-Rin that she knew that she was not an ordinary maid ever since she saw her change on her very first day. Then, she says that she does not know who sent Mey-Rin though and offers her triple of Mey-Rin's current income if she changes sides and works for Heathfield. After all, it would be very inconvenient if she destroyed the manor and the blood supplies.