Here follows my description/commentary/whatever about the plot of Kuroshitsuji Myu. Naturally this contains HUGE SPOILERS so if you're trying to wait until the DVD comes out to hear about the plot, then for goodness' sake don't read this post! (That means you, besto friendo.)
Also, I swear the plot is more coherent than I make it sound in this summary >_< My Japanese is pretty good but Kuromyu has a history of confusing me. And no one says "OMG" in Kuromyu. My retelling of the plot is incredibly juvenile and leaves out quite a few details.
Okay, so the basic plot is as follows: there's been some serial murders in London that Ciel has been asked to investigate. The Shinigamis are also looking into the case, because the souls of the murder victims weren't properly collected. William totally thinks this is all Grell's fault, but Grell is like "Nope, wasn't me!" So now both Ciel and the Shinigamis are trying to find out who's been killing people and stealing their souls.
Also, two of the Shinigamis are original characters for the musical, Eric Slingby and Alan Humphries. Alan has something wrong with him (I didn't quite catch what, it wasn't a word I recognized, but some sort of disease maybe?) that is causing him a lot of pain, and Eric is trying to watch out for him and take care of him. At one point fairly early on, Eric and Alan get into a fight with Sebastian, but it all turns out to be a misunderstanding or something, and Alan's disease/whatever is hurting him so Ciel lets them go to Phantomhive Manor so Alan can rest.
Meanwhile Ciel and Sebastian have been investigating the murders, and Sebastian figures out that the killer is actually Eric. There's a confrontation about this and Sebastian ends up stabbing Eric with Grell's chainsaw, causing some Cinematic Record to spew out of him, but he doesn't actually die. The Cinematic Record shows Eric committing the murders. And Alan is all "Eric why did you do that :<" and Eric is like "Uh. I'm not telling, bye >_>" and runs off. He ends up running into the Viscount of Druitt, and they sort of become a team (the Viscount is kind of enamored once he sees that Eric is able to kill people without leaving any physical marks on them). It turns out that Eric is sort of hoarding the souls of the people he's killing, and is trying to get a thousand, though the reason for that isn't made clear for a while, I don't think. So the Viscount is like "OMG I will totally get you a ton of people to kill, just leave it to me, this is gonna be awesome!" And Eric is like "I really hate this actually BUT I MUST DO IT >:|"
The Viscount decides to host an opera, where only women and children are allowed to come as guests. Men can come too, but only if accompanied by a woman (sounds kind of like a host club XD). Ciel and company catch wind of this, and that's when the cross-dressing party happens! Ciel, Grell, Alan, and Abberline & his assistant all end up in dresses, because they sure don't have enough ladies to go around. So they all go to the opera, trying to find Eric and stop him before he can kill all the guests ("I have to talk to Eric! :<" says Alan in his pretty white dress).
Side note: The opera singer did a fantastic job, I was very impressed with her voice. Unless she was just lip-synching or something, but I'm pretty sure that was real.
Eric totally kills most of the guests, though. Eventually Alan finds him and is like "Eric why are you doing this! :< If you need souls so badly, just take mine!" And he's about to stab himself with his own deathscythe but then Eric stops him all "No! You can't! I'm doing this for you!" Because it turns out that 1,000 souls are needed to cure whatever it is that's wrong with Alan, so Eric has been collecting souls in secret to cure him. Alan and Eric make some sort of promise to each other after this and take off their Shinigami glasses (I didn't really get what was up with this scene either, honestly ^^;) and run off. Eventually they have to stop because Alan is in too much pain to keep going, so Eric helps him sit down and then starts killing more people to try to collect all the souls he needs, and then Sebastian finds him. They start fighting, because Ciel ordered Sebastian to kill Eric, and during the fight it's revealed that Eric has collected 999 souls and only needs one more in order to cure Alan. Then Eric sees Ciel and is like "Oh hey! There's a soul right there!" and is about to kill Ciel, but Alan tries to stop him because he doesn't like that all these innocent people are being killed. So Alan rushes in front of Ciel and gets hit and killed by Eric's deathscythe.
Then there's a Cinematic Record scene that has a reprise of the song Alan and Eric had sang earlier about loneliness and flowers, and also they talk about being partners and stuff, it's super-heartbreaking given that one of them just accidentally killed the other. T_T
The Cinematic Record scene ends and Eric is holding Alan's lifeless body in his arms. Sebastian says that now Eric has collected the last soul and has a thousand, but it was all for nothing. Eric breaks down and asks Sebastian to kill him, because he can't live with himself after killing Alan. Sebastian stabs Eric with Alan's deathscythe and he dies. (Note: I was totally crying by this point, like the huge dork that I am.)
After Eric dies, all the souls that he was collecting are released, and float through the air like snowflakes. They do all get officially collected, though; William and Ronald make sure of that.
Then there's sort of an epilogue scene where Sebastian remarks on how much he's looking forward to eating Ciel's soul one day - he's anticipating it so much that he didn't even bother trying to eat any of the thousand souls that were all released at once.
And I think that was about where it ended! I admit I was very distracted by Uehara Takuya's presence and paid less attention to the plot than I should have XD And I left out quite a few details, but I think I got all the important things in. All in all I really enjoyed this myu, though I should have known better than to get attached to the original characters :P