In 2365, she came aboard the USS Enterprise-D on an urgent mission to intercept the Klingon sleeper ship IKS T'Ong, which was about to become active. The vessel would have been unaware of the Federation-Klingon alliance, therefore posing a threat to Federation establishments in the Boradis system. While searching for the T'Ong, she renewed her relationship with Worf, but refused to marry him when he asked her. Due to this short renewal, however, she gave birth to a son a year later. On the mission, she tried to persuade Captain Picard to destroy the Klingon ship as soon as possible, but thanks to an idea of Worf's, this could be prevented: to convince the T'Ong crew of the Alliance, Worf posed as the captain of the Enterprise-D, with K'Ehleyr as his first officer. After the T'Ong crew had agreed to lay down their weapons, K'Ehleyr beamed aboard the T'Ong to prepare the crew for the 24th century while waiting for the IKS P'Rang. Before transporting over, Worf told her that he would never be complete without her. (TNG: "The Emissary")
In 2367, she accompanied Klingon chancellor K'mpec, who had chosen Picard as Arbiter of Succession, to a meeting with the Enterprise-D. When she came aboard, Lt. Worf learned of his son Alexander for the first time. K'Ehleyr expressed her desire to become Worf's mate at this time, but he refused, saying that he did not desire to share his discommendation with her and Alexander. After K'mpec's death, she assisted Picard in the Rite of Succession, briefing him about Klingon rituals and tradition. As she therefore had much influence on the Rite, Gowron tried to bribe her with the command over a Klingon ship or a seat in the Klingon High Council, but she refused. Later, she discovered evidence that proved Duras was involved in the conspiracy that led to Worf's discommendation; on learning of her search through attempts to access Klingon High Council records he sealed, Duras attacked K'Ehleyr in her quarters and killed her. Worf transported to Duras' ship shortly later and killed him in vengeance. Afterward, Worf claimed Alexander as his son and placed him in his adoptive parents' care on Earth. (TNG: "Reunion")
Though K'Ehleyr knew much about Klingon culture and fighting techniques, she never showed much respect for Klingon values, which was sometimes a cause of great fury for Worf. This disrespect may have been caused by her feeling that she got the worst of her parents' attributes – her mother's humor and her father's temper – which got her into plenty of trouble. Consequently, she never taught Alexander anything about the Klingon way of living before her death. (TNG: "Firstborn"; DS9: "Sons and Daughters")
Background information Edit
According to a reference cut from the script of "The Emissary", K'Ehleyr met Worf on Samrin's Planet in 2359. When they were reunited six years later, K'Ehleyr was disappointed at how much Worf had changed since their initial encounter.
Ronald D. Moore commented regarding K'Ehleyr's death: "I am happy to pass the buck on this one to Michael Piller. It was his idea to kill K'Ehleyr during the story break because it would be a great dramatic turn and would provide Worf with ample reason to go stick a bat'leth into Duras' guts." (AOL chat, 1997)
"kay-LAHR" was the pronunciation of this character's name from the script pronunciation guide for "The Emissary". 
A subtle reference is made to K'Ehleyr in the Q Continuum series, which features the female Q, another character played by Suzie Plakson; when the two are first introduced, Picard finds her features slightly familiar, but cannot place exactly where he "recognizes" her from. Additionally, in the Star Trek: Voyager episode "The Q and the Grey", the same female Q, when told off by B'Elanna Torres over an engineering problem, says she's always liked Klingon females as they are "spunky."
In the Pocket TNG novel Diplomatic Implausibility places K'Ehleyr's grave site as Woodlawn Cemetery in New York City; by coincidence, Lt. Marla Aster, whose son Jeremy Aster became a member of Worf's family after her untimely death, is buried there as well.
The mirror universe novel Rise Like Lions introduces K'Ehleyr's mirror universe counterpart; the result of her mother being captured and brutally raped by several Klingons before she was rescued, K'Ehleyr joined the Terran Rebellion, becoming first officer of the Rebellion's Enterprise under the mirror Picard (K'Ehleyr reasoning that the crew would not be comfortable with even a half-Klingon in a position of total power over them).