This was the first Klingon space vessel ever mentioned on Star Trek. While unseen during the original broadcast of "Errand of Mercy," it was later established as a D7-class warship in the 2007remastering of the episode.
The English script in the Klingon language edition of the comic "Against Their Nature" establishes this ship as the IKS Vortha, a ship of a class of Klingon scout vessels, similar in design to the D7-class but about one third the size.
Soon after, eight Klingon space vehicles, led by CommanderKor, engaged the USS Enterprise, driving it away from the system before they assumed orbit around the planet to deploy their invasion forces.
The two fleets were later prepared to engage each other outside of Organia, but were stopped by the Organians, who made all instruments of violence on both of the star fleets radiate a temperature of 350 degrees, effectively ending the battle before it began. (TOS-R: "Errand of Mercy")
This fleet was unseen in the original broadcast, but was established as being comprised of D7-class warships in the 2007 remastering of the episode.
In the original broadcast of this episode, the exact classification of this vessel was obscure. Kras identified his ship as "a small scout ship," but the ship encountered by the Enterprise, which may or may not have been the same vessel, was identified by Pavel Chekov as a "Klingon warship" and was referred to in the false distress calls sent by the Klingons as a "Klingon vessel."
In the 2007 remastered release of this episode, the ship was recreated as a D7-class warship. According toMike Okuda, "The story situation required the ship to be very small on the screen, and we wanted it to be recognizable as a familiar Klingon ship. Small changes to the design would not have been noticeable, and larger changes could run the risk making the ship unrecognizable, especially since we only saw it for a few moments."
In recalling the original appearance, the script refers to the visual as "a throbbing point of light pinpoints the Klingon vessel. (Similar effect as used in "Mudd's Women".)" and "the pulsing light that marks the Klingon vessel." Visually, this effect appeared as a flat shape with some sort of "handle" affixed to it.
The vessel was detected by the Enterprise arriving at Neural just as the Enterpriselanding party was beaming up to the Enterprise. Because the Enterprise was on the far side of the planet, the Klingon vessel did not spot the Federation ship, which was confirmed when the Klingon's routine message to their home base made no mention of Enterprise. To prevent detection by the ship, the Enterprise maintained radio silence, and after beaming a second landing party down, it left orbit to hide from the Klingon vessel until the Enterprise landing party could complete its mission. (TOS-R: "A Private Little War")
This vessel was merely mentioned in the original version of the episode and remained unseen until its appearance in 2008 as the recurring D7 CGI model, for what was its final Star Trek remastered television appearance.
After Kryton sabotaged the warp drive of the Enterprise, it was the job of the battle cruiser to strafe the Federation starship in order to persuade it into engaging its warp drive and destroy the ship.
The ship was ultimately outmaneuvered by the Enterprise and limped back to Klingon territory. (TOS-R: "Elaan of Troyius")
This episode was the earliest in the original run of the series to be filmed using the "D7" studio model. However, it was not the first appearance of the ship on the air, as "The Enterprise Incident" (see below), an episode produced after "Elaan of Troyius", originally aired in September of 1968, while "Elaan of Troyius" would not air until December of that same year.
All appearances of this type of ship in the remastered series before "Elaan of Troyius"—which aired prior to "The Enterprise Incident" in the inaugural run of the remastered series—represent new scenes added using a computer-generated model. None of those episodes featured this type of ship or its model in their originally-aired versions. (A similar addition was made for the Star Trek: Deep Space Nine episode "Trials and Tribble-ations"; much of the episode takes place during the events of "The Trouble with Tribbles", but that episode as originally aired never showed the Klingon ship described within it, and a new scene was created for the Deep Space Nine episode featuring a computer-generated model.)
In the original broadcast, all three Romulan ships were of the "D7" type and were represented by the same studio model. In the remastered version of the episode, the Romulan Bird-of-Prey replaced one of these ships, and the Romulan D7s sported markings on their ventral hulls in the image of birds – resembling the markings on the Romulan Bird-of-Prey – that were not present in the original broadcast.
As "The Enterprise Incident" aired before "Elaan of Troyius" (the first episode produced using the "D7" studio model) during the original run of the series, "The Enterprise Incident" featured the first on-air appearance of this type of ship.
During a brief engagement, the Enterprise disabled the propulsion units of the lead cruiser, and the power supply of the second cruiser, leaving the crew unharmed. Following the engagement the Romulan cruisers to moved off, the latter, according to Spock needed several days to complete repairs. (TAS: "The Survivor")