SG-1: Arthur's Mantle

Season 9, episode 18 starts out with Colonel Samanth Carter working in her lab on an Ancient device found in Glastonbury when Dr. Daniel Jackson, Teal'c and Col. Cameron Mitchell discovered the truth about the mythical Avalon in the season première. As she tries to calibrate something a bright light shines out and she and Cam disappear from our screens. And so another crazy Stargate adventure begins! :o)

Sam out of phase!

It turns out that the device has shifted Sam and Cam into another dimension (apparently there are 11 of them according to string theory, don't ask!), slightly out of phase with ours. This means they can see and hear everything, but can't touch anything or communicate with anyone as no one can see them! 

Cameron: "That was an alternate reality, this is an alternate dimension. Hell, all I need is a good time travel adventure and I've scored the SG-1 trifecta!"

Dr Lee tries to figure out what happened to Sam and Mitchell (Sam and Cam was sounding weird, I'm changing) after he and Daniel realised they simply disappeared from the security video footage of the lab after a weird staticky outburst when Sam did something to the machine. Daniel's the one to actually realise what happened to them once Dr Lee mentiones lepton radiation because pretty much the same thing happened to him in the season 3 episode Crystal Skull (definitely one of the weirder episodes!).

Daniel goes through the texts from Glastonbury and finds a mention of Arthur's quest to recover 13 sacred objects from the Underworld, one of which was a cloak, the Mantle of Arthur, said to render the bearer invisible! The texts say it was kept safe by Myrddin a.k.a. our "Ancient" friend Merlin! 

So it turns out the Mantle of Arthur is a device, not a cloak, built by Merlin probably so he could access the device and do his work on it safely in another dimension, away from prying eyes.

Daniel discovering Merlin's secret

After Dr Lee screws up Daniel is also phase-shifted and he can finally read the display in Ancient that only Sam and Mitchell could see in the other dimension. This allows him to discover Merlin's big secret: he retook human form (was an Ascended Ancient) so he could develop a weapon capable of killing ascended beings... like the Ori! He did this because the Ancients refused to worry about an Ori invasion of this galaxy. After completing the weapon he relocated it to another planet...

Daniel phase shifts them back, and voilà! Except that it doesn't turn out so well for Mitchell who had gone offworld with a team to see what had happened to Teal'c and the Sodan (who had called asking for help after being attacked by one of their own). He gets phased back at a very dangerous moment, thankfully Teal'c is on hand to bail him out! :p

Elements from Arthurian Legend

• Myrddin -> a.k.a. Merlin. Here confirmed to be an ascended Ancient who retook human form (and guided Arthur and his knights) so he could develop a weapon to kill other ascended beings in preparation for what he considered the inevitable Ori attack.

• Arthur's Mantle -> One of the 13 Treasures of Britain (see below), the name should indicate a cloak worn by Arthur, which according to legend would render its bearer invisible, but nope it's an Ancient piece of technology that phases you into another dimension, cool! :p

• Thirteen Treasures of Britain -> here simply called "thirteen sacred treasures" which Arthur went to get from the Underworld. They are mentioned in some of the earliest Arthurian legends of Welsh origin.