Fred called her to re-ensoul Angel, being the only person that has ever done it (well alive person). During her little trip Willow and Fred engaged in mild flirtation, her and Wesley had a talk about going "dark" and some little lines were thrown in as Aly and Alexis are now married, were dating then, Cordy wrapped Connor round her finger some more.
Also Willow displayed mega mojo again, she figured that the jar holding Angel's soul may have been captured but was still a jar. So she sent a little magical marble thing to go and break the jar. However evily possessed Cordy stopped it until Connor startled her and it broke the jar, re-ensouling Angel.
Then Willow and Faith went off into the sunset (well Sunnydale anyway).
Also in that ep Faith and Angelus went into Angel's past (well it was Faith's dream) and we saw how Angel arrived in America, how he saved a little puppy dog and how he fed off someone who was dead, which led to him feeding on rats again in the 80s/90s.
