I have only seen a handful of Dragon Ball Z films so far, but the structure is already becoming glaringly obvious. A lighter first half is followed by a quick introduction of the episode's evil, after which an epic battle ensues. Or, at least, a battle that is supposed to be epic, as I consistently find these the most boring parts to sit through. Tree of Might is no exception.
The Dragon Ball crew is enjoying some off-time, having a little camping trip in the woods. Little do they know an alien gang is targeting the Earth. They send a small seed our way, which quickly turns into a malicious tree that saps all the life force out of our precious planet. It's up to Gohan and friends to make sure the aliens don't win.
Though I clearly prefer the lighter side of Dragon Ball, it is mostly just by comparison, as watching some weirdo fighters camping in the woods isn't all that interesting either. Compared to the endless battles (now I'm mortally hurt, the next moment I'm even stronger than before, ad infinitum) it's not too bad though. The only really sad thing is that the animation quality is taking a small but definite dip with each new film. Still not great, but not quite as bad as I'd feared it would be.