Spike and Drusilla are introduced in the second season. The definitely add a bit of spice to the series.
Spike and Drusilla are a happy couple who have come to Sunnydale to find the Hellmouth—it tends to be the siren call to all evil creatures. Their hope is that they might find a cure for Drusilla's weakness. Eventually they do, and find it in a book that was taken from Giles' library. I'm not sure how they knew what to look for, but they do have one of their minions purloin the book.
It culminates in episodes 9 and 10 “What's My Line” Part One and Part Two.
Within the book is the incantation and rite to bring back a vampire's strength. Thing is, it's difficult to translate unless you have a key. Thus, during the next few episodes, they eventually figure out where to look. Eventually, through Drusilla's ability to read Tarot cards that seem to have figures and such that tell her what exactly she needs to know, they go to a mausoleum and find this key. It is actually a reliquary where religious items were kept of the very author of this book they found. Odd that this author would be found right there in Sunnydale California, but we'll give the writer a break on this.
When they figure out that she needs her maker, who is Angel, then we get to the nitty gritty on this. They are help;ed out by Slayer #2, Kendra, who shows up in town sent because her Watcher has been notified that the last Slayer was dead (Buffy: “I died, just a little bit”), and was sent to the hot spot of Sunnydale because of the Hellmouth activity. And together they join forces to save Angel from Spike and Drusilla who initiate the deadly ritual which will bring Drusilla's strength back.