@Roaster A lot of stuff was ambiguous but I'll do my best to give a plot summary: Narue is a Lovecraft style cosmic entity. Saki's grandmother summoned Narue during a festival 100 years ago. She made a deal with Narue, in which Narue would save grandma's village in exchange for grandma sacrificing her lifeblood to open the gate, and promising future blood to return Narue home.
Narue lived up to her end, spending the last 100 years sharing technology and knowledge with the villagers. The village is now safe. During that time, she met and formed a romantic relationship with Saki. But the gate to Narue's homeworld is only open during the festival every 100 years, so Narue must now return home. It is ambiguous whether she is obligated to return as part of her deal with Saki's grandmother, or if she simply does not want to wait another 100 years for the gate to reopen. Regardless it is Saki's role as grandma's descendant to sacrifice her lifeblood to get Narue home. As a farewell they go on one last date, and Narue expresses her gratitude for the human name Saki gave her. Then she sacrifices Saki, uses her blood to open the gate, and returns home. Saki, beheaded and dying, is amused that she wore managed to wear a yukata after all.
@feha Regarding the last line about a yukata: I think this is wordplay, since the Japanese 着る usually means "to wear" but can more broadly mean "to bear (something) on your body." Since Saki's body has become the bridge to bear Narue home, she is sort of "wearing" Narue's yukata as well. I could be completely off-base on this though; its a weird line and I'm unsure if I'm correct.