Fairy Tail Volume 4 by Hiro Mashima
After dealing the destructive consequences of their last mission (the Fairy Tail team took out a lot of buildings and railroad tracks resolving the Lullaby plot line), Natsu, Gray, and Lucy try to relax back at the guild hall. Natsu sends Happy the flying cat up to the second floor to steal an S-class assignment from the upstairs workboard. Those missions (and indeed, the second floor itself) are only for S-class wizards in the guild, a status to which Natsu aspires. How can you gain S-class ranking without performing an S-class job? Also, the guild is known for its looseness about following rules, so he should be okay, right? Natsu drags Lucy into the mission and Gray tags along. The task is to end the curse on Galuna Island. The natives turn into horrible creatures at night, especially under the moonlight. They assume their curse is related to the moon (which looks purple from the island), so their request is that the Fairy Tail team destroy the moon. The team wisely decides to investigate the rest of the island first, in case some other cause is responsible or another solution is available (i.e. possible).
The story is an interesting mystery that opens up some of Gray's history. The ultimate cause of the problem turns out to be more than just the moon's weird color. The manga is still very faithful to the television series with some fun supplemental material at the end.