On stardate 42606.5, the USS Enterprise arrives at Starbase 179 to transfer a group of personnel assigned to an officer exchange program. Temporarily joining the Enterprise is Ensign Mendon, a Benzite who is eager to please.
Wesley is the first one to greet Mendon warmly, initially mistaking him for Mordock, another Benzite who co-tested with Wesley last year at the Academy. Wesley has been rather lonely without his mother and is still feeling very lost, so the familiar-looking friendly-seeming Benzite is a welcome sight for him. But Mendon gently lets Wesley know his name. Wesley is slightly embarrassed and disappointed, but when Mendon gently reassures him that it's a common error, Wesley feels more at ease and continues to make Mendon feel welcome.
Captain Picard informs his secondâinâcommand, Commander Riker, that Starfleet would like an officer to join the exchange program aboard a Klingon vessel. Riker agrees to the assignment, and begins preparing. Riker talks to Lieutenant Worf about the details of Klingon protocol mentioning a rumor that one of the duties of the first officer is to assassinate the Captain. Worf confirms stating that if the captain becomes weak and unable to perform, it is expected that his "honorable retirement" be assisted by his first.
Soon the Enterprise meets up with the IKS Pagh, an old Bird of Prey. Riker heads to the transporter but before he goes, Worf hands him a small transponder, mentioning that it is merely a security precaution. Ensign Mendon studies a strange spot on the port side of the Klingon ship's hull but doesn't bring it to Picard's attention.
Riker meets Kargan, the captain of the Klingon ship. When Riker's authority as new first officer is challenged, he is forced to attack his subordinate to prove his strength.
Back on the Enterprise, Worf alerts Captain Picard of an unknown substance on their hull which is identified as a lifeform reacting with the hull. Ensign Mendon steps up and indicates he had found a similar spot on the Klingon vessel and Picard admonishes him for not informing anyone sooner. Mendon states his analysis wasn't completed and that aboard Benzite ships, it is improper to report it until then. The Enterprise then sets off to find the Klingon vessel to inform them of the problem, while the crew works to find a way to decontaminate the hull.
Meanwhile, Wesley senses that Mendon is a tad embarrassed at having been dressed down a bit and walks over and quietly reassures Mendon that things are all right, that everyone has days like this in a fashion very similar to what Beverly Crusher would do. Mendon smiles at Wesley thankfully.
Back on the Pagh, Kargan shows Riker a schematic of the ship and the brown spot that is growing on the side of the bow. Riker admits that he has never seen anything like it before and suggests corrosion or a meteor impact. Kargan replies their science station has determined it to be an organism eating the hull and in eight hours the entire ship will be compromised (as the Klingon vessel is smaller and comprised of weaker alloys than the Enterprise). Kargan harshly believes it came from the Enterprise and hands Riker a tactical report showing that the Enterprise conducted an intensive scan of the ship in that area. Kargan demands to know what type of weapon was used to damage the hull where Riker insists the Enterprise has no weapon that would cause damage like this. Riker asks why would they attack with him on board, but Kargan says "why" is no longer important and they must respond. He orders to engage the cloaking device and sets a course to intercept the Enterprise.
The ships eventually meet. Worf tries hailing the Pagh, but gets no response with Data suggesting the Pagh is either cloaked or destroyed. On the Pagh, Riker suggests the Enterprise may be here to help, but Kargan instead asks for the surest method of attack against them. Riker refuses to divulge secrets of Starfleet stating he swore a vow to serve the Klingon ship but he cannot break a vow he swore in the past. Kargan gives an approving nod and tells Riker if he had divulged those secrets he would have labeled him a traitor and have him killed on the spot. On the Enterprise, Mendon informs Picard that he found a way of cleaning off the organisms and Picard has him relay the procedures to the Klingons.
Back on the Pagh the communications officer informs Kargan that the Enterprise says they can clean the organisms off the hull where Riker says it's proof they are here to help, but Kargan stays the attack and has torpedoes readied to fire on his command. Back on the Enterprise, Data indicates that a lack of debris must mean that the Pagh is cloaked and as a precaution, Picard orders Red Alert.
The Klingons detect the raise of shields where Riker indicates is standard procedure when entering a suspicious situation, but Kargan remains unconvinced. Riker then recommends they fire at 40,000 kilometers, it will reduce the Enterprise's response time. Kargan smiles seeing Riker is fulfilling his oath and tells Riker to give the firing order. Riker accepts, but states Kargan's reasons for forcing this attack are not valid. As the distance is approached, Riker draws the signaling device Worf gave him earlier and Kargan asks if it is a weapon. Riker hands it to him and he unwittingly triggers Riker's emergency signal which Worf picks up on his console.
Worf informs Picard who orders the shields to be dropped so Riker can beam back to the ship. On the Pagh, there is a moment of stunned surprise when the Enterprise shields drop. Seeing his chance, Kargan orders to decloak. Suddenly, Kargan is beamed off his bridge and materializes on the Enterprise bridge. Kargan reacts with fury knowing Riker has tricked him. He draws his disruptor but Worf quickly hits him with a stun beam from his phaser knocking Kargan to the deck.
Riker explains to his crew that as first officer, he has relieved the Captain because he was acting in an irrational manner. Riker takes Kargan's place in the center chair and orders the ship to decloak. Klag fears they will be destroyed, but Riker says if they do, then he'll die with them. Klag drops the cloak and the Bird of Prey appears before the Enterprise. With all the Pagh's weapons locked on the Enterprise, Picard calls the Klingons and tells them they are here to assist. Picard then hears Riker respond with an order to lower shields and surrender. Picard then bears an odd smile and tells Worf to lower shields and "surrender".
The Enterprise cleans the Klingon ship, while Kargan is returned to the Pagh and orders Riker to leave.