Episode 12:
Willow discovers on which plane Giles is returning, and attempts a spell to stop him. The plane does indeed crash, but Giles is lucky to survive. Meanwhile, Xander and Anya return from their honeymoon, and Buffy tries to discover where Willow is hiding.
Episode 13:
Oz puts in an appearance and is horrified to find that Willow has turned on her former friends. Blaming himself, he goes out to search for her, against the express desires of the others. Turning into his wolf form when night comes, he is captured by Willow and imprisoned.
Episode 14:
When Oz does not return, Buffy deduces that Willow must have captured him, and vows to rescue him. Giles is brought to Sunnydale in the aftermath of the plane crash. Buffy and Dawn go to meet him and fill him in on everything that has happened. Reluctantly, Giles reveals that the Watchers' Council has declared Willow a tremendous threat and that she must be destroyed.
Episode 15:
Xander and Anya find that holy matrimony isn't all smiles and happiness when a demon sent by Willow possesses Xander and gives him superhuman strength in order to kill Buffy. While possessed, Xander injures Anya severely. Giles and Buffy use a spell to restore Xander to normal, but Anya finds it hard to forgive him.
Episode 16:
Giles deduces that the only person to have had contact with Willow and not been injured during recent events is Angel, and that he must be spoken to, as he might have valuable information of Willow's whereabouts. Buffy goes to LA to find Angel, leaving the others in Sunnydale at Willow's mercy.
Episode 17:
When Buffy locates Angel, he turns out to be uncommunicative, after the events of Episode 10. It eventually turns out that he has no information and that he hasn't seen Willow again. He does express deep regret for his rash actions, but Buffy harshly tells him that that won't bring Spike back. She returns to Sunnydale, to find that Willow has been on the rampage. Giles' house is destroyed and everybody is missing.
Episode 18:
Willow has captured Giles, Xander and Dawn, and imprisoned them in the dungeons of her hideout (a magical castle which she has conjured up herself) with Oz. The four of them begin to plan an escape, but Willow has been eavesdropping on their plan. She takes Xander to a torture chamber and begins to slowly kill him. Meanwhile, Buffy locates Anya, left for dead in the rubble of Giles' house. Now that Xander is gone, she realises how much she loves him and can forgive him for Episode 15.
Episode 19:
Willow continues to torture Xander, as Buffy and Anya contact the Watchers' Council for advice. Buffy is sceptical of receiving their help, but when they are able to recommend a spell that will locate Willow for her, she realises that they do have their moments of usefulness.
Episode 20:
Buffy and Anya cast the location spell, which reveals Willow's castle and makes it visible to all. Attempting to break in, Buffy is repelled by a magic barrier. However, Anya has seen barriers like this before and helps Buffy to destroy it. Once inside, they begin the search for Willow.
Episode 21:
This episode is basically calm before the storm. Buffy and Anya hunt around the castle for Willow, eventually stumbling into the dungeons to rescue Giles, Dawn and Oz. Alerted to Willow's torture of Xander, Buffy heads straight for the torture chamber to rescue him.
Episode 22:
Buffy storms into the torture chamber and finds that Willow is not there. Xander is, however, and Buffy releases him, telling him to flee with the others. As he and Anya are joyfully reunited, Willow materialises in front of Buffy. A massive showdown ensues, in which Buffy is finally victorious, but only just. Willow lies dead on the ground, and the demon that tempted her finally appears. Buffy manages to kill it also, and thus its most evil deeds are undone. Willow and Spike return to life, Willow as her former self.
Okay, maybe that's not a very good conclusion, but it's the best I can think of. I'm sure that, if they were to work off this framework, Joss Whedon and his team could come up with something pretty cool.