Following the events of Baptism Of Fire, Tiffany returns from a holiday with no apparent memory of being a vampire slayer the morning after Daria accidentally lets slip that she is in fact Daria Morgendorffer, not Joy Merryweather.
it is eventually revealed that Tiffany, along with a lot of other slayers that have been dissapearing, were being committed to nut houses because their parents were being influenced by the First Evil, and when the First Evil's involvement is revealed, it all kicks off with the first evil using Bringers to steal and deploy weapons from US Army arsenals.
The Watcher's Council is under seige as it is suspected to be a terrorist organisation, and after the first evil recreate's Tara's death in view of police snipers, the 22nd regiment, in a near repeat of Operation Nimrod, are sent in to clear house.
Meanwhile, Daria, grouped with Quinn, Sandi, Tiffany, Tiffany's parents, Jane, Amanda and Robodaria, in a stolen SWAT APC, find themselves under nuclear attack 15 miles out of Lawndale, they erect a mystical shield Daria had researched since Amy's vampires started manufacturing fusion only devices, which allowed them to survive being right next to the hypocentre of a 15MT explosion.
Daria works on Tiffany, her condition is unusual among the Slayers they extract from the mental health facilities, and they get part way to cleveland (killing a large fromation of Bringers and capturing their weapons) Daria, with the assistance of Amanda and Quinn, try to get into Tiffany's head.
This goes wrong because Tiffany somehow hijacks Amanda, and traps everyone in there.
Unable to free themselves, and with no one able to free them externally, Sandi and Jane erect the same kind of shield as what was used earlier to protect Daria, Quinn, Amanda, Tiffany's parents, and Sandi, and Robodaria takes over, using recently macguyverred UAVs and GPS units to direct the counterattack against the inbound bringer forces, they then clear Cleveland of brigners and set up a defensive perimiter.
The Watcher's council, having escaped their london base, manage to take a Hellmouth (there are apparently a lot of these aroudn the world) and the SAS, having learned that they had been fooled and that the Watcher's Coundil are in fact good guys, start working on the european theatre, even though the SAS had managed to kill a number of Watchers, Slayers and witches (Including Kennedy and Giles) they work cohesively and then assist Robodaria in the US theatre in preventing the First Evil's XO (or whatever role Caleb filled before Buffy split they guy with a weird mystical axe with a stake pummel) from opening the Cleveland Hellmouth.
The oxygen within the shield around the APC gets used up to the point where Tiffany can no longer maintain control over everyone's minds, but at this point she has spent hours detaining Quinn's consciousness and raping that consciousness in a simulation of a past trauma Tiffany herself had suffered when she was still a potential slayer.
The First Evil's XO, Gabriel Kent, is finally killed by Robodaria after she grabs a 1T(TNT equivalent) nuclear rocket off him that he stole off of Illyria who had come with Angel Investigations who had just killed the circle Of The Black Thorn which had caused an army of demons to be unleashed in LA, and then the US and UK Governments get Buffy, default leader of the Watcher's Council following Gile's death, to agree to merge forces with them.