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Tear Jerker: Stellvia of the Universe
Uchuu No Stellvia takes place in a world where war is such a distant memory that most of the teenage main characters don't even know what it is. This makes it all the more tragic when a confrontation with alien spacecraft turns suddenly, devastatingly deadly. The slow, haunting rendition of the opening theme playing in the background — a more upbeat version of which previously played in-show during moments of triumph — doesn't help. Neither does finding out later that the aliens were actually trying to help humanity, and only killed in self-defense. If the Foundations hadn't fired first, no one would have died.
The entire last episode, but especially when Stellvia goes into the fracture.