I'm writing this on August 2, 1998. Last viewing: at least 6 months ago.
I don't recall this one as well as the Lum movie I saw more recently (Beautiful Dreamer), but I do remember liking it more.
Again, I'd already seen a couple of episodes so I already knew the characters a bit. This one actually involved a rather interesting plotline, and had some beautifully animated sequences.
Basic plot: Town, centered around a great tree. It's in bloom, first time in years. This tree must be over 10 stories tall. Odd things start happening in the town, and the tree seems sick. Eventually we discover that the town itself is, in a sense, alive, and is having an immune response to an alien influence: Lum. This is a reasonably interesting idea, and it was presented with sufficient drama and suspense to be a nice plot.
A reasonable choice if you're in the mood for a really nice, solid anime.