The blood of all Klingons has become water. Since the Federation alliance we have turned into a nation of mewling babies. Romulans are strong. They are worthy allies. They do not turn Klingons into weaklings like you (Worf)!
~ J'Dan after being exposed as a spy.

J'Dan was a Klingon villain in the Star Trek: The Next Generation episode The Drumhead.

J'Dan was a member of the Klingon Defense Force. Am exobiologist he held the rank of Lieutenant. He was also a spy for the Romulan Star Empire, believing them to be potentially strong and worthy allies to the Klingons. J'Dan saw the alliance with the Federation as weakening the Klingon Empire, turning Klingon blood into water.

J'Dan suffered from Ba'ltmasor Syndrome, a disease that affected some Klingons which required regular injections of medicine.

In 2367 J'Dan saw an opportunity to advance Romulan interests when he accepted a post on the USS Enteprise (NCC-1701-D) in the officer exchange program between Starfleet and the KDF. He used his Ba'ltmasor Syndrome as a cover to bring his own special hypospray on board. This hypospray was modified to read Starfleet computers and transform the information it read into inert protein sequences. These proteins were injected into unsuspecting individuals. After these individuals left the Enterprise J'Dan's Romulan handlers intercepted the unfortunate individuals, killed then, and extracted the information from their bodies. The Romulans were able to among other things obtain a schematic of the Enterprise dilithium crystal chamber as a result of J'Dan's work.

J'Dan was able to work undetected for several weeks. However his espionage came to an end when an explosion destroyed the ship's warp core hatch. Security quickly identified him as a person of interest and had him confined to quarters. J'Dan implied to Worf - who at that time was ostracized from Klingon society due to his father being condemned as a traitor - that he would intercede on Worf's behalf with his friends if Worf would help him escape from the Enterprise. When they reached J'Dan's quarters Worf struck J'Dan and told him that he would find out how he damaged the warp core, and that they would return J'Dan to the High Council who would put him to a slow death.

A few days later Admiral Norah Satie and Sabin Genestra helped the Enterprise crew question J'Dan. After Worf confronted him with the evidence he had gathered, J'Dan admitted that he was a spy for the Romulans. Captain Picard informed Worf that he could take J'Dan into custody.

Following his arrest for spying, engineering regained access to the warp core, and found that the explosion had been purely accidental - that a design flaw in a new hatch put on the core after the Borg attack of 2367-68 eventually caused its failure.

J'Dan was returned to the Klingon Empire, and put on trial for his crimes. Supposedly the Klingon Empire put J'Dan to the slow death that Worf promised they would.