J'Dan was an influential member of the Klingon Defense Force with the rank of lieutenant and served as a scientist in exobiology in the 24th century. As his career advanced, he began to see the Klingons as weak and their "life blood as thick as water since the Federation alliance and have become a nation of mewling babies". He also saw the Romulan Star Empire as "strong and worthy allies who do not turn Klingons into weaklings". He then began secretly spying on the Klingon Empire and sending information to the Romulans on their behalf.
In 2367, J'Dan saw his opportunity to steal Federation secrets when he served in a scientific exchange program. He was assigned to the Federation flagship USS Enterprise-D and remained there for several weeks. He gathered information by using his disease, Ba'ltmasor Syndrome, as an ideal cover for using a micro-replicating hyposyringe (used to give him treatments) as a secret optical chip reader (which was specially modified to read data on Starfleet isolinear chips) and download information from the Enterprise's computers, and then secretly inject passing guests aboard the ship unknowingly with the information, now encoded into the form of an amino-acid sequences and carried in the bloodstream and appear as inert proteins on most forms of scanners. When the people departed, J'Dan informed his Romulan contacts where the person was going and they intercepted the person, killed him, extracted the information and put it into their own computer system for any reason they saw fit. One such person was a Tarkanian diplomat who passed through the ship and was injected with information, left for the Cruces system and disappeared, presumably killed by J'Dan's Romulan agents.
When a subtle manufacturing flaw caused the Enterprise's dilithium chamber hatch to explode, disabling the ship's warp engines, an investigation into the accident centered on J'Dan. He initially attempted to keep quiet, but when confronted by overwhelming information gathered by Lieutenant Worf that pointed to his guilt, he gave up and proudly declared his links to the Romulans and his belief that he was a patriot who acted for his peoples best interests. This caused him to lose his influence and was sent back to Qo'noS. His fate is unknown (presumably put to a "slow death" for spying).
The extent of the information he stole is unknown, but among the most significant things stolen was the schematic drawings for the Enterprise's dilithium chamber, which came into Romulan possession one week of his being on the ship. (TNG episode: "The Drumhead")