The Threshold began their careers as profiteers and used their transport window capabilities to carry out important missions for their clients.
The Threshold eventually came under a contract to the Lobri, a race of spider-like, parasitic entities who were trapped on a plane of reality that held together the link between human minds. The Threshold supplied the Lobri with the resources required to instigate a breakout, a large gathering of humans at the Notting Hill Carnival, and later supplied them with three of the Doctor's previous companions, Peri, Ace and Sarah Jane Smith. The Lobri fed off the emotions of the humans and this allowed them to gain enough power to escape from their dimension. (COMIC: Ground Zero)
The Threshold were later hired by the Time Lords to stop a Dalek plan to open a doorway into an alternate universe to destroy a group of alternate Daleks that had crossed over into the main universe. This would then leave the Daleks free to conquer the Multiverse. The Threshold were stopped by the Eighth Doctor who also stole the Threshold's payment from the Time Lords: a black box containing the Seal of Rassilon which would allow the Threshold to communicate with any species in the universe. (COMIC: Fire and Brimstone)
The Threshold was later destroyed by the Eighth Doctor, who sent his companion Shayde into the Threshold base Wormwood to find the source of their power. The Doctor landed the TARDIS in the main control room of the base and used a device known as the Eye of Disharmony to wipe out the Threshold's agents. The Doctor then destroyed the Eye of Disharmony, causing Wormwood to be completely destroyed, killing Abraham White and the Pariah in the process. (COMIC: Wormwood)