Jenny was young and pretty, (and hipper than any other teacher I've EVER seen). Giles loved Jenny, even when she dragged him to a Monster Trucks show. Giles and Jenny seemed great together until...
We learned that Jenny was actually "Janna" a Romanian Gypsy. Janna had been sent to Sunnydale to 'watch' Buffy Summers (Sarah Michelle Gellar), the Slayer. And to destroy Angel (David Boreaneaz), Buffy's vamp boyfriend. (In the episode "Becoming" we also find out that Ms. Calendar is a distant relative of the Vamp, Drusilla. They both came from the same Gypsy family, that put the spell on Angelus.) The rest of the gang soon discovers this and they completely cut Jenny out of their little 'circle'. Giles is now depressed, and broken-hearted. And the rest of the kids are just angry. The only one who seems to still really like Jenny and talks to her is now Willow Rosenburg (Alyson Hannigan), Buffy's best friend.
In an attempt to regain the groups trust, Jenny tries to conjure up a new soul for Angel (after he loses his in a 'moment of complete happiness' with Buffy, due to a curse by Janna & Drusilla's family). She figures it all out, but she just has to translate the incantation. She goes the whole 9 yards, she buys an 'orb of Thesila' and uses her computer to finish the spell.
Jenny does finally translate it and she saves it onto a disk. Angel destroys everything else.
Before this however, Giles has reconcilled with Jenny, once she explains that her duty to her family was "all [she] was ever taught."
Jenny Calendar died in the episode, "Passion," soon after saving the spell. Angel killed Jenny at Sunnydale High School, he broke her neck, he got in because of a sign outside, "Enter all ye who seek knowledge". Miss Calendar's computer was smashed and print out destroyed. He didnt get The Disk. Angel took Jenny and left her body in Giles' bedroom; it was one of the saddest moments ever. Giles sought revenge, and burned down the factory where Angel, Dru & Spike lived, with Buffy's help. We see that everyone still cares for Jenny by the way that they act when she dies, they break down, including Buffy. Willow took over Miss C's class.
At the end of "Passion" Buffy and Giles are at the cemetery, looking at Jenny's gravestone, and Buffy apologizes for not being able to kill Angel, she explains that she is ready now. (Buffy eventually "kills" Angelus, by sending him to Hell, but not before Willow could fufill Miss Calendar's dying wish, to transform Angel back into a 'good guy'. Angel is now back in Sunnydale, after an extended stay in Hell).
(Jenny reappeared in the episode "Becoming", but it was only a hallucination of Giles' brought on by vampire Drusilla.)
(Jenny also made a "comeback" in "A Buffy Christmas", as The First Evil. All those that Angel killed were coming back to haunt him, and Jenny was the "leader".)
Now, other than 'visions' of Jenny, the gang still speaks fondly of her now and then. And the horrible neck-snap is now immortalized in the BtVS opening credits.
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