Kargan was initially suspicious of Riker, and made Riker swear an oath of loyalty to the Captain and his ship. However, after a subatomic bacteria was discovered eating away at the hull of the Pagh, Kargan was convinced it was part of a Federation plot. He decided that his only course of action was to attack the USS Enterprise-D, which he blamed for putting the bacteria on his hull.
Attempting to take advantage of Riker's oath to him, Kargan demanded to know the secrets of the Enterprise-D, but Riker refused to tell him. He respected this, telling Riker that if he had told him the secrets he would have killed him where he stood.
Despite this, Kargan refused to listen to Riker, who tried to tell him that the Enterprise-D was not to blame. Riker then gave Kargan an emergency transponder he had smuggled aboard. Thinking Riker was holding it, the Enterprise-D beamed Kargan over. Kargan then materialized on the bridge and immediately pulled out his disruptor, but was stunned by Worf.
After Riker had made contact with the Enterprise-D and the Pagh was repaired, Kargan was returned to his ship. He struck Riker for his insubordination, and ordered him off the ship. (TNG: "A Matter Of Honor")