Colonel Grat was a Tandaran military officer in charge of Detention Complex 26, a prison on a moon in Tandaran space, where dozens of innocent Suliban were being held during the Tandarans' conflict with the Suliban Cabal.
In 2152, a shuttlepod carrying Enterprise NX-01 officers Captain Jonathan Archer and Ensign Travis Mayweather was shot down by Tandaran ships when they accidentally entered the moon's military zone. They were detained and questioned by Grat, and although he believed they were not affiliated with the Suliban, he could not release them, citing strict regulations preventing him from doing so.
Grat told the two Starfleet officers that they would appeal to the Tandaran Central Magistrate for their release before Grat could let them go, and that they were not permitted to contact Enterprise. He stated that a transport would arrive to take them to Tandar Prime in three days, but until then, they could not leave or communicate outside the detention complex.
Later, Grat contacted Enterprise using an untraceable triaxilating channel and informed the crew of the situation. He also told them they could not speak with the imprisoned officers, but sent the ship coordinates for Tandar Prime so that they could attend the hearing of their crewmates.
Having learned from Archer that Enterprise had prior encounters with the Suliban Cabal, Grat took it upon himself to contact Tandaran intelligence, who relayed to him information regarding the events surrounding the incident at Broken Bow, including the Cabal's attempts to capture a Klingon named Klaang, Archer's contact with a former Cabal member named Sarin, and his encounter with Silik. Believing Archer had valuable information on the Cabal and who they were working for, Grat brought the captain in for questioning. The captain refused, however, and instead asked why the Tandarans had detained Suliban who were not members of the Cabal.
Grat claimed that the Suliban were imprisoned for their own protection, to prevent them from becoming innocent casualties of war and to ensure they could not be recruited into the Cabal. Grat then persisted in questioning Archer for further information on the Cabal and his involvement in the Temporal Cold War. When Archer stood his ground, Grat threatened him with a longer stay in the detention complex.
Grat later contacted Enterprise to inform them that their crewmates' hearing had been "postponed." This time, however, communications officer Hoshi Sato was able to isolate Grat's frequency and T'Pol ordered Enterprise to proceed to the location of the detention complex.
When Enterprise assumed orbit around the moon, they were threatened by Grat. However, he could not prevent Archer and Mayweather's escape, along with the eighty-nine detained Suliban. (ENT: "Detained")
Later that year, while vacationing on planet Risa, Archer suspected that a disguised Tandaran named Keyla was an agent of Colonel Grat sent to retrieve information from Archer. Keyla denied the accusations, claiming she had never even heard of Grat. Before Archer could question her further, she knocked him unconscious and escaped. (ENT: "Two Days and Two Nights")