Hey shoppingbags... major spoilers if you haven't seen the previous series follow!
George dated Nina as they both worked at the same hospital but she had no idea he was a werewolf. Meanwhile, Herrick, who is in next episode, was organising all the vampires whilst holding down a job high up in the police. There was a conspiracy where police used vampires to tidy up cases, e.g. get rid of repeat offenders whilst simulataneously covering up for vampire messes/feeds. Herrick goes way back with Mitchell, from when he was seriously bad and lived for the kill. Mitchell went 'good' and split off from Herrick but they were still on speaking terms, Herrick trying to lure Mitchell back to the blood. Meanwhile Herrick was orchestrating a plan to enslave humans as a food source for vampires, and so Mitchell decided he had to die, and George agreed to help. Herrick was thrown into George's werewolf holding room with George as he changed, Nina burst in but was dragged out as the door slammed, but George caught her with his claws. And that's how Nina found out her boyfriend was a werewolf and got turned, and how Herrick died.
Mitchell then took up where Herrick left off as the vampires needed organising to stop uncontrolled killing sprees happening that would expose vampires to rest of society, but he failed to convince them to give up blood. One girl, Cara, killed two teenagers and was condemned to death with her teeth pulled out by Mitchell, but he secretly didn't go through with it and left her shut in a dungeon instead. Mitchell enlisted another 'celebrity' vampire to give up blood to encourage the others to stick with it, but a human cult type-thing blew up their meeting, killing this celeb vampire (whose name I forget). Mad with grief and out for revenge, Mitchell and the celeb vampire's wife/girlfriend, Daisy, attacked and killed the passengers on the "Box tunnel" train. Mitchell was then wracked with regret, and gave up his position as leader. Cara and Daisy tried to revive Herrick by spilling their blood on his grave... and evidently it worked...