Buffy The Vampire Slayer (1997) - 4x11 - Doomed

Buffy and Riley have to work out their relationship given their new understanding of what they do. An earthquake rocks Sunnydale and Buffy worries that it could signify the end of the world. They find out about a group of demons who are in town collecting sacred items which together have the power to end the world.
When the plans of the demons are uncovered, the gang must return to their former sanctum inside the dilapidated Sunnydale High School (the library) to stop the Hellmouth from opening.