Okay, from the link above, we're getting a bigger idea about the plot...
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The U.S.S. Defiant from the Original Series episode "The Tholian Web" is deposited in the 22nd Century of the Mirror Universe thanks to the spatial interphase it was trapped in. An idependent salvage ship staffed by Gorn and other aliens recovers it and steals the technology on board. Later, that ship is captured by the Enterprise, which up until recently was commanded by Captain Forrest before First Officer Archer killed him (shades of how the mirror Kirk killed the mirror Pike to take command of the Enterprise). Archer's crew tortures one of the aliens for information on where the Defiant is, but he doesn't talk. They torture a second and Archer threatens to use a phaser taken from the Defiant (unique in that it can vaporize someone, whereas phase pistols can't) on him to get him to talk. That one leads him back to the Defiant, where there's a Gorn on board.
Later, an Imperial ship called the Avenger, commanded by an Admiral Black and with Science Officer Soval, is trying to fight off some Andorians. Archer and the Defiant shows up to bail them out. Black gets interested in the ship from the future and hatches a plan to take it for himself...
Is it just me, or doesn't this sound a bit too...ambitious? Modern Trek shows have shyed away from doing episodes involving things from TOS (namely, the bridge of the old Enterprise) because of the cost involved, and when they did do it, they figured out how to do it without building a lot of the old sets and relying heavily on green-screens. Not only are we supposedly getting an old-style Constitution-class starship in this episode, but we're also getting Mirrorized versions of the ENT sets and wardrobes, a full-blown Gorn, and also a live Tholian. Given how much ENT's budget has allegedly been slashed, I'm willing to bet that it will end up not being the Defiant but another ship, or there won't be any scenes shot on it.