The U.S.S. Enterprise arrives at a subspace communications relay station near the Klingon border on a resupply mission. However when an away team boards the relay there is no sign of the two officers assigned there, Lieutenant Aquiel Uhnari and Lieutenant Rocha, and the station's shuttlecraft is not present. While searching the station the away team find a stray dog which belonged to Lieutenant Uhnari. The away team also find a substance on the floor which Dr. Crusher determines is a type of cellular residue. In addition the station's shuttlecraft is missing.
During the course of the investigation the crew uncover evidence that a Klingon was on the station leading Beverly and Will Riker to suspect that Uhnari and Rocha may have been the victims of a Klingon attack. Geordi backs up this theory when he examines Uhnari's personal logs. He finds an entry in which Aquiel relays her fears to her sister Shianna about a Klingon named Morag. Picard contacts the local Klingon governor, Torak, and learns that Morag is commander of one of the Klingon ships that patrols that section of the Klingon border. Torak informs Picard that he will look into the matter personally.
The senior staff meets with Torak, and he produces Aquiel alive. She explains that Rocha attacked her and that her last memory was escaping from him. She doesn't remember precisely what happened. To help clarify what really occurred, Picard requests to speak with Commander Morag, the Klingon who was allegedly harassing the station. Attracted to her, Geordi befriends Aquiel, and takes her to Ten-Forward. He reveals to her that he surveyed her logs and personal correspondence as part of their investigation. He then asks her about Rocha, and she tells him that she didn't like Rocha but she didn't wish him dead. She realizes, however, that the senior staff of the Enterprise suspects her of murder.
Meanwhile, Beverly continues to examine the cellular residue found on the deckplate, while Riker and Worf, who are examining the shuttlecraft, come across a phaser, set to kill. Given this new development, Riker and Worf question Aquiel; however she still insists that she did not kill Rocha. Geordi sits in on this interrogation, and is sympathetic towards his new friend.
Commander Morag then arrives aboard the Enterprise and meets with the senior staff. He admits that he was present on the station, and that he took priority Starfleet messages. Geordi returns to the station and discovers that Rocha's personal log has been tampered with. He confronts Aquiel who admits deleting messages from Rocha's log, because Rocha, as the senior officer, was going to declare her insubordinate and belligerent to Starfleet. Scared that this new evidence will condemn her as Rocha's killer, she agrees to stay aboard the Enterprise because Geordi has faith in her. He and Aquiel use an ancient method of her people to bond and share their thoughts.
While Beverly examines the DNA found on the deck plate yet again, the material moves and touches her hand. It then forms a perfect replica of her hand. Due to this, she suspects that the real Rocha may have been killed by this strange coalescent organism, and a replica of him may have attacked Aquiel in search of a new body. Believing that the organism is now Aquiel, Riker and Worf race to Aquiel's quarters and stop the ritual she is conducting with Geordi, believing she was about to attack him.
With Aquiel in the brig the Enterprise proceeds to the nearest starbase to turn her over to the proper authorities. Geordi is in his quarters along with Aquiel's dog reminiscing about her. Suddenly, the dog begins to transform right in front of Geordi - the dog is really the coalescent creature. The creature blocks the doorway preventing Geordi from escaping. Instead he reaches for a phaser and fires at the creature, vaporising it. Later Geordi explains to Aquiel, who has been released, that Rocha was replaced by the organism. When it attacked her and she managed to get away the creature turned to the only other life form on the station, her dog.