Complex sentence: The core four share a moment talking about going to the mall after saving the world which causes Giles to say "the earth is definitely doomed," echoing the end of the second episode of the first season of Buffy.
Split into simpler sentences: The core four share a moment talking about going to the mall after saving the world which causes Giles to say "the earth is definitely doomed". Thus echoing the end of the second episode of the first season of Buffy.