The scene opens up to the familiar; it is April 5th, 2063 in Montana. Just as in Star Trek First Contact, the Vulcans have landed and Cochrane meets them. After an awkward attempt to do the Vulcan greeting, Cochrane reaches into his coat and pulls out a shot gun and mortally wounding the Vulcan. The people then charge toward the Vulcan ship to take it over.
The introduction of the show shifts to an opening that tells us we are in the mirrorverse. In this universe, the Terran Empire has conqurored many different alien cultures and the ISS Enterprise is its pride and joy. Captain Forrest is in command with Jonathan Archer as First Officer. The crew is essentially the same except for being more cruel and harsh.
The empire is in the middle of a bloody war with its neighbors and rebellious subjects. The war is not going well for the empire and they need something to give them an edge. This something comes through Archer who has discovered that the Tholians have some kind of technology, not only from the future, but from another universe. Forrest however thinks going into Tholian space is too risky.
Archer throws a coup and imprisons Forrest and sets course into Tholian space. The mission goes well thanks to a Suliban cloaking device installed on Enterprise, but it has its problems thanks to a saboteur. A Tholian scout ship discovers them and attempts to signal for help. The exotic creature is captured however and killed by Dr. Phlox when it proves too dangerous to keep alive.
Eventually T'Pol leads another coup to reinstate Forrest as captain. Archer is surrounded and gives up, but not before revealing that the ship is programmed to continue to it's designation and course cannot be changed. Forrest has Archer disciplined for the remainder of the journey.
The Enterprise arrives at an asteroid field where a secret base is hidden. They scan the base and discover what had come from the alternate universe. It is the star ship Defiant. Forrest decides to send an away mission to the Defiant and see what could be salvaged. Archer is put in command of the mission, but T'Pol is secretly told to kill Archer.
The away team is beamed aboard the Defiant and a quick survey is done. They are amazed at the sophistication of the ship, but more pressing danger takes place. The Enterprise has been discovered and the Tholians surround her. Forming an energy field around her, they destroy Enterprise. Archer and the away team just watch in shock on the Defiant's view screen.