The second volume opens just a few moments after the cliffhanger that ended volume one. The stalemate between Ouga and Ura is broken when Ouga loses control of his power, and the darkness begins to eat him. When she sees Ouga going under, Yayoi immediately rushes to his rescue, trying to pull him out. This is something of a surprise to Ura and Hakuyoh, especially since Ouga had just kidnapped her. Yayoi yells at Ura, asking him if he wants to regret not trying to save his brother for the rest of his life. Ura pitches in and helps pull Ouga up out of the darkness.
While Ura and Yayoi are trying to haul Ouga out of a pit of darkness there’s an explosion that knocks Yayoi out. Someone turns up from the city and retrieves Ouga. This person is apparently Ouga’s teacher, and he seems to have it in for Ura, because he states that “this time” he isn’t going to do anything to Ura. Then he heads off with Ouga in tow, and the only way we know who this guy is, is because there is an author’s note about him. We get no resolution concerning this guy or Ouga’s fixation on proving himself to Ura and his father. (I really hate it when there are loose threads. They drive me right up the wall.)
Yayoi is still out of it, and Hakuyoh urges Ura to absorb Yayoi’s powers and kill her. Ura is strangely reluctant to do this, even if it will save his father’s life. He hesitates long enough for Yayoi to wake up and bind him again with the Cat God’s Rosary. He’s turned into a cat, something with surprises Hakuyoh. (Hakuyoh had though Ura’s human form had been his sealed form, he hadn’t seen Ura get turned into a cat.) After some discussion where Yayoi assumes that Ura and Hakuyoh will go home now, but Ura wants to stay with her (because he doesn’t want anyone else to steal her powers).
So, Ura and Hakuyoh both end up back at the temple where Yayoi lives. Ura becomes obsessed with finding Yayoi’s “weakness.” At first, Hakuyoh thinks that this means Ura is still planning on killing Yayoi, but he’s very disappointed to discover that Ura seems happy to just play practical jokes on her. Yayoi goes between seemingly oblivious kindness toward both Ura and Hakuyoh, to wary defensiveness and back to oblivious kindness in her interactions with Ura and Hakuyoh, which pretty much baffles both Ura and Hakuyoh.
A little later, Yayoi ends up at a beach trying to find and exorcise a spirit. While she’s there, another demon turns up, trying to drain her powers. Ura defends her, but somehow ends up getting possessed. (Yes, a demon got possessed by another demon. Do not ask me how that works, I don’t even know.) The being possessing Ura pretends to be Ura, but Yayoi sees right through him. The spirit tries to attack her through Ura, but she frees Ura from the binding. She actually offers to let Ura kill her so he can defeat the spirit, but he refuses. He apparently has acquired feelings for her, and he does not want to.
Ura manages to defeat the spirit on his own, but is drained of his powers (temporarily) as a result. He falls asleep with his head in Yayoi’s lap. This ends the story, though there’s an extra story involving time travel at the end where Yayoi is getting sick from having been infected with darkness, and a second story about Ura when he was a child.
I liked Mugen Spiral for the most part, though I had a problem with the way so much was left unresolved at the end of the story.