Terra Prime is a Xenophobic Terrorist Organization bent on "Liberating" Earth of alien influence and isolating Earth from the rest of the Galaxy, They became very prominent in the mid-22nd century after the Xindi attack, In 2155 they hijacked the Verteron Array on Mars, The Leader Declared that unless all non-humans leave Earth immediately, He will fire the Verteron array directly on Starfleet Command, he was thwarted by the Crew of the S.S. Enterprise NX-01.
Here's what I picture Happening in Castaways:
After these events Terra Prime went back to underground status and rumors eventually started circulating that the group have disbanded. This would not prove to be true as a few members were still out there as of the 23rd century, one of them was on the Starship Grace where they tried to goad the temporally displaced Command crew to join their cause due to the fact that as of their time Earth was politically divided and they thought they could use that to their advantage, but when that failed they, in their single-mindedness, forgot John Frederick Paxton's mistake when he made the Binary Clone of T'Pol and Charles Tucker III, but this time they used the DNA of Dom Oranzi and T'Kara. There are also Terra Prime "Sleepers" in the Federation Council who will be activated and kill the alien council members.
The Storyline will combine elements from the storylines of the first season of "Agents of S.H.I.E.L.D." regarding the H.Y.D.R.A. Infiltration and of course the Terra Prime Story Arc from two of the last episodes of Enterprise entitled "Demons" and "Terra Prime"
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Star Trek Castaways and all related elements (c)