Klaang was a Klingon courier in the service of the Klingon High Council during the 22nd century. In his adulthood, Klaang was seven feet tall and became a primary protagonist of the Broken Bow Incident.
In 2151, Klaang met with a Suliban female named Sarin on Rigel X, where Sarin provided him with proof that the Suliban Cabal had been attempting to destabilize the Klingon Empire by staging attacks, making it appear as if one House was attacking another. To do this, she covertly injected the information directly into Klaang's DNA, so that it could be safely transported back to the High Council.
Upon leaving Rigel, Klaang was pursued by two Suliban soldiers attempting to retrieve the evidence, forcing Klaang to crash land his vessel on Earth, in the town of Broken Bow, Oklahoma. Klaang was eventually successful in killing the soldiers by luring them into a grain silo and destroying it with his disruptor, but sustained serious injuries himself when he was shot with a plasma rifle by a farmer named Moore, on whose land Klaang had crashed.
Klaang was subsequently transported to Starfleet Medical in San Francisco, where he was put on life support and treated by Doctor Phlox. Soval, the Vulcan ambassador to Earth, recommended disconnecting Klaang from life support and allowing the Vulcans to transport his body back to the Klingon homeworld, Qo'noS, but was overridden by Starfleet Admiral Forrest, who agreed to let the crew of Enterprise NX-01 return him alive. En route to Qo'noS, however, Enterprise was boarded by a group of Suliban soldiers, who abducted Klaang and transported him to a helix, where he was interrogated by Cabal leader Silik. Klaang was later rescued by Captain Archer and returned safely to the Klingon High Council Chamber, where the proof given to him by Sarin was extracted from his blood, averting a Klingon civil war. (ENT: "Broken Bow")
Less than one month later, Sub-Commander T'Pol mentioned Klaang twice while trying to explain to Vorok – the commanding officer of a Klingon battle cruiser who was threatening to destroy a Xyrillian starship – that the Klingons were in Archer's debt since he had returned Klaang to the Klingon Empire. (ENT: "Unexpected")
Later in 2151, Commander Tucker reminded Captain Archer that, if Enterprise's launch had been delayed any longer than it had been, Klaang's body would have been sent back to the Klingon homeworld "in a box", a fact that Archer claimed he kept reminding himself of. (ENT: "Silent Enemy")
News of Klaang's flight from two Suliban soldiers and of how he had eventually returned to his homeworld aboard Enterprise eventually reached the Tandaran intelligence agency. This knowledge was utilized by Colonel Grat in a conversation with Captain Archer.(ENT: "Detained")
When Archer was transported back in time to the day before the Broken Bow Incident, he realized he wasn't dreaming as he still knew about Klaang and Phlox; he also used this knowledge in a discussion with T'Pol in 2152. (ENT: "Shockwave")
While deliberating how to deal with a band of Klingon marauders under Korok, Archer suggested to ask the High Council for help since they had returned Klaang and helped the IKS Somraw. (ENT: "Marauders")