Commander Kaagh, also called Kaagh the Slayer, was attached to the Tenth Sontaran Battle Fleet's Special Assault Squad. He was the lone survivor of a failed invasion of Earth by the Sontarans. Also, he was the first Sontaran to be known to act on the behalf of another species, serving the Bane as a mercenary.
Luke Rattigan destroyed the Sontaran flagship. Kaagh's space pod was sent off course. It crashed on the planet Earth. Despite damage to the ship and exposure to G-force which might have killed him, Kaagh survived, suffering a large scar down the right side of his face and a similar scar on his right hand and losing a piece of his right ear.
As a member of the Special Assault Squad, Kaagh's purpose was to infiltrate deep behind enemy lines to predict and anticipate their movements. Kaagh hoped that by doing this he could make up for his fleet's failure to conquer Earth. He began a covert attempt at destroying Earth and making himself a hero: "Kaagh the Avenger", then "Kaagh the Destroyer".
Kaagh chose to use a nearby radio telescope to control the many satellites orbiting Earth and send them plummeting into all the nuclear power plants. He distracted Professor Nicholas Skinner and his daughter Lucy and put them under his control with neural control implants; he wanted the Professor to act solely on his orders, while Lucy would be a sleeper agent to help him infiltrate potential enemies who might stop him. Soon after, Sarah Jane Smith, Luke Smith,Maria Jackson and Clyde Langer arrived to investigate the "lights" in the sky. These were actually drones Kaagh had used to lure out the Skinners.
Kaagh watched from a distance, then confronted Luke and Clyde when they found his cloaked pod. However, they evaded him and brought Sarah Jane and Maria back to investigate. Kaagh revealed himself, his past and his plan of vengeance and hatred for the Doctor. Thinking two steps ahead of them, Kaagh pretended to pursue when they ran away, but soon followed them back to the telescope, where he found Sarah Jane about to disable the professor's implant with her sonic lipstick. Kaagh learned Sarah Jane was a good friend of the Doctor. He planned to take her back to Sontar to have her be punished in the Doctor's stead, after he had avenged his fleet.
However, the children escaped and Kaagh was forced to hunt for them. He finally gave up and returned to the telescope to continue his plans. As he had expected, Kaagh captured Clyde and Maria when they tried to cut the power. However, Luke surprised him by deactivating the signal to cause the end of the world. Kaagh activated Lucy's implant and initiated Plan B. However, Chrissie Jackson arrived and stabbed his probic vent with her high heels, knocking him out and electrocuting her unconscious. After he woke, Kaagh was forced off Earth at gunpoint. Sarah Jane felt he would return.
Too ashamed to return to Sontar, Kaagh worked as a mercenary for the Bane. They had found the consciousness of Horath and used Kaagh as a go-between for its transportation from planet to planet. A few weeks after this he met Mrs Wormwood, a member of the Bane family cast out for her actions. He learned there was a high price on her head, but she persuaded him to help her control the universe, which would be more fulfilling than simply serving the Bane. They plotted to become emperors of the galaxy.
Mrs Wormwood travelled to Earth, closely followed by Kaagh, who came in during a siege by the Bane Kindred. He found Mrs Wormwood and killed her attackers. They stole Luke and the Tunguska Scroll, which would show them where Horath was buried. Luke escaped, but Kaagh caught him. However, Mrs Wormwood was growing tired of him.