The Butler, Delivery is Chapter 159 of the Kuroshitsuji manga.
Sometime after Haku ordered to kill Jan and Hao and took in Mey-Rin as their "brother," he brings her to her first mission: She has to assassinate Yang, a high-ranking official of one of their rival groups—Gelaohui—because said group has started to sell opium however they pleased in their—Qīng Bāng's—territory. If Gelaohui were to find out that Qīng Bāng killed Yang, it would initiate a war between the two groups. Because of that, Haku has chosen Mey-Rin for this job.
Haku takes her to a building opposite Yang's residence and on the other side of the Thames. There, he gives her a rifle and two choices: Mey-Rin either kills Yang or flees and dies like her friends did. Haku assists her with the rifle while she shoots Yang.
Ten years later in the Limehouse District in London, two Qīng Bāng members talk about their latest mission and about Mey-Rin who was, at some point, given the nickname "Owl." Mey-Rin herself sits empty-eyed against a wall and holds her rifle. The two men eventually try to trick her: One of them hands her his shoe, telling her that it is something good to eat. The trick works, and Mey-Rin bites into the shoe.
The men laugh at her mistake and are promptly silenced by a busy and stressed Haku. Haku has been in a terrible mood lately because the new pharmaceutical laws and restrictions imposed on the opium trade by the Crown and the government are giving Qīng Bāng financial problems.
Shortly afterward, someone comes in and informs Haku that they received a message from headquarters: A top official—Lau from the Shanghai branch—will be sent to Great Britain. Haku blames the Queen's Watchdog for the new regulations and Lau's transfer and assigns Mey-Rin to hunt down and kill Ciel Phantomhive.