Claude has kidnapped Yin and plans to sell her to the highest bidder, who turn out to be some old Chinese friends of his. When Hei rolls in to rescue her, Claude disguises himself as her to trick him, then disguises her as himself so that he electrocutes her. This completes her awakening into Izanami, who begins sucking up the souls of thousands of contractors, until the Syndicate steps in, pacifies and re-captures her. Depressed over the loss of his love, Hei takes up drinking, and is then confronted by Madame Oreille.
Watching it now, a year after it aired, we get a similar sense of this Gaiden series that we did with the Star Wars prequels, particularly Episode II with regards to this episode. That is the sense that everything in these four OVAs was inevitable. We saw the entire Ryuusei no Gemini series, so we knew what was going to befall Hei, Yin, et al. There’s not much in the way of surprise of novelty in the plot, as it’s merely filling in the blank between the two animes. So it comes down to execution: and it was executed pretty well.
There was also a sense that something good was gradually becoming corrupted, namely Yin. One new fun fact we did learn is that the entire reason the Syndicate was founded was to prevent a prophecy from coming true – that is, the Awakening of Izanami/Yin. That failed, but the Syndicate had long since forgotten its purpose anyway. Yin, it would seem, was a prisoner of fate; a puppet to events that had a mind of their own. Her love of Hei, and his of her, got pushed to the side. Despite growing to love and need one another so deeply, they were never meant to be. Which sucks…and also explains why Hei is such a drunken mess at the beginning of Gemini.