“Endless Terror” is episode one of the fifth and final season of Warehouse 13.
In the season four finale, “The Truth Hurts”, Claudia had discovered that her supposedly dead sister’s DNA had been used to keep her intact in “All the Time in the World”. Artie had known said sister was alive since before meeting Claudia herself, and Claudia did not take this well. Artie said Claudia’s sister, Claire, was very dangerous – given what Artie does, this is not a promising sign.
Things between them might have deteriorated further if other factors had not intervened (although the matter crops up more than a few times during the episode). Paracelsus, who had been captured (probably what he planned for) after he was made immortal, playing on Pete’s fears regarding Myka – he was saying that Myka’s cancer was much worse than she had claimed – had managed to dupe Pete into freeing him. It was obvious to the observer that this was a trick, but not to Pete. With Paracelsus in the Warehouse, he was able to seize total control of it. Claudia had severed Mrs Frederic’s link in order to save her, and remained behind to confront Paracelsus as Artie, Pete, Abigail and Mrs Frederic were forced out.
The episode opens with Claudia confronting Paracelsus. Claudia is being protected by the Warehouse, and it won’t let Paracelsus kill her. Unfortunately, Paracelsus, being in control of the Warehouse, also has control over her, so he is able to defeat her and then get her knowledge in the same way that he got knowledge from Nick in “Lost & Found”. He is going to use Claudia to help him combine different artefacts to build something new.
Pete is collecting Myka from the hospital. The surgery was a success, as the tumour was benign, according to her. Pete doesn’t immediately fill her in on what has happened at the Warehouse (he does eventually explain what happened). Outside the Warehouse, Artie is covering a car in purple goo so that Jinks will be able to breach the barrier that Paracelsus has erected around it. Artie says that this will work. It does not. Mrs Frederic is trying to meet with the Regents, but they are on retreat. Inconvenient timing. Myka has another idea to break through the shield, but Artie is trying another method with Jinks using a lot of stolen car batteries. This also doesn’t work. Jinks suffers again.
Apparently, after Sykes used the shield against them in “Stand”, the Regents had installed a failsafe. Which, it appears, only Myka bothered to read the addendum in the manual about. Artie says he’s backlogged since he turned evil and tried to poison the world. So they get in quite easily. Which meant that Steve didn’t need to crash a car into the shield or get blown back by electricity. Paracelsus still has control over Claudia and uses her against them.
Control of the Warehouse was not Paracelsus’ ultimate goal, but only a step along the way. What he actually intends is something rather different. He goes back into the past and changes the present of the Warehouse. In the new present, one with a far more advanced, and far nasty, Warehouse, Paracelsus has been duplicating artefacts and using them to create supersoldiers. They have to find out what Paracelsus did in the past and fix the timeline. In the new Warehouse, there are familiar faces but in different roles.