This episode marks the first time that a full Tholian is seen on-screen in any Star Trek series. The upper part was based on the Tholian as seen on the viewscreen in The Tholian Web (1968). The concept of Tholians requiring very hot environments was continued from Future Tense (2003), which had based this on the fan theory that the viewscreen in The Tholian Web seemed to ripple from heat. The creators of 'Enterprise' took some creative license with the lower part of the body, giving it six spider-like legs; the reasoning was that since the Tholians catch their enemies in an energy web, they might look a bit like spiders.
The teaser features archive footage of Star Trek: First Contact (1996). Additional footage of the first contact between humans and Vulcans in the Mirror Universe was shot on traditional film instead of with digital cameras, so that the new footage would blend in seamlessly.
The opening title sequence of this two-parter features a different title music and some new footage depicting the history of Human warfare and interstellar domination of the Terran Empire. The original idea was to feature more episodes inside the Mirror Universe, so that this new title sequence could be reused, had "Enterprise" not been canceled.
The USS Defiant is the same starship that disappeared in The Tholian Web (1968). When mirror Archer and his team board the Defiant, the captain and crew of the ship are found in the exact same positions as they were in The Tholian Web (1968).
A mirror-universe story for Enterprise was originally conceived as a way of bringing William Shatner into the series' fourth season. The Mirror Universe of Captain Kirk would have been revealed to have been sent back in time by way of the Tantalus field and ended up being the cause of the split between the two universes. As he and Paramount were unable to reach an agreement, the plans to have Shatner included in the series were discarded.