(covers information from several alternate timelines)
In 2268, on Deep Space Station K-7 while posing as an assistant to Nilz Baris, the Federation Undersecretary for Agriculture, Darvin poisoned a shipment of quadrotriticale grain, which was destined for Sherman's Planet. Shortly after, Darvin's true identity as a Klingon was exposed by Captain James T. Kirk, using a tribble. (TOS: "The Trouble with Tribbles")
After being disgraced, Darvin was abandoned by the Klingon Empire and wandered from place to place posing as a merchant named Barry Waddle, eventually becoming trapped on Cardassia Prime during the Klingon-Cardassian War. While on Cardassia, Darvin learned of the Orb of Time and plotted to use it to change his past and gain revenge on Kirk. His opportunity came when the USS Defiant was assigned to transport the Orb back to Bajor in 2373.
As Waddle, Darvin gained passage aboard the Defiant and used the Orb of Time to transport the entire ship back to 2268, where he planted a tricobalt bomb inside a tribble in the storage compartments aboard K-7. The crew of the Defiant successfully prevented the bomb from killing Kirk, minimizing the changes in the timeline. Darvin was captured and returned to Federation custody as well as his past self getting caught again. (DS9: "Trials and Tribble-ations")
Background information Edit
Arne Darvin was played by Charlie Brill.
Though the Defiant crew continually referred to him as "Arne Darvin", this name is most likely a pseudonym, and not his real name (since Klingons do not generally have two names). It is clearly established that he was surgically altered to appear Human.
The comic "Beneath the Skin" established that the real Arne Darvin was in fact a Human who was killed by the Klingon Intelligence operative originally named "Gralmek". (The Klingon's orders had been to stab the real Darvin in the back, but Gralmek insisted on facing his counterpart openly — reasoning that a man has the right to know his killer.) Gralmek was a Klingon with ridges, however he was a mach ghoti', a Klingon of diminutive stature and strength, which made him an ideal candidate for surgical alteration to appear Human. The comic also established that Gralmek was the first, and following his failed mission, last Klingon to undergo such alterations for the purposes of espionage. As well as back-story on Gralmek prior to becoming Darvin, the comic established that after the tribble incident Gralmek was held in Federation custody for several months before being returned to the Klingons in a prisoner exchange. Following this he was discommendated and expelled from Klingon space, which led to his desire to somehow get revenge on Kirk and redeem himself for the Klingon Empire.
The novella "Honor in the Night" depicts an alternate universe where Arne Darvin successfully poisoned the quadrotriticale, which was eventually shipped to Sherman's Planet; the resulting ecological disaster cost thousands of lives, which sickened him greatly. Ashamed at what he had done, he dedicated the rest of his life to restoring his honor and atoning for his crimes. In this story, Darvin's real name was "Krek" and he had always been a smooth-headed Klingon. Arne Darvin was a false identity and there was never a human with that name.
In Star Trek: Federation - The First 150 Years, the real Arne Darvin was a human who was traveling to Earth to begin training in the Federation Diplomatic Corps. He was beamed off his ship by a cloaked bird-of-prey and replaced by a surgically altered Klingon agent named "Kron". The real Darvin was executed immediately afterwards.