Spock's father, the great Vulcan diplomat Sarek has arrived on board the Enterprise with his second human wife, Perrin. His mission is to lay the foundation for trade relations between the Federation and the Legarans, after which time he will retire due to his old age. Sarek surveys the conference room set up aboard the Enterprise, but expresses apprehension and annoyance when several key decor elements are not in place. Captain Picard assures Sarek that everything will be in order before the Legarans arrive.
He also reveals that he has prepared a full welcoming ceremony for the ambassador, including a chamber music concert in that includes a performance by Commander Data. Sarek's two aids indicate that he would prefer to dispense with formalities and ceremony and be led straight to his quarters, as his old age makes him more prone to fatigue. Disappointed, Picard nevertheless understands the situation. Instead, he visits the ambassador's quarters and invites Perrin to the concert, inadvertently annoying Sarek who has trouble meditating in the next room.
Later that evening, Picard is surprised to see that Sarek has decided to accompany his wife to the concert, despite previous suggestions otherwise. All is well until Commander Data's musical performance overwhelms Sarek with emotion, prompting him to start crying. Perrin notices this, and the two leave the concert after having inadvertently drawn the attention of Picard and Deanna Troi who are now quite concerned that a Vulcan could be prone to such intense emotions.
Almost immediately, strange events start occurring. Wesley Crusher and Geordi La Forge become embroiled in a heated argument while setting up the conference room, forcing Commander Riker to intervene. Later, Beverly Crusher expresses severe anger at Wesley for being late to arrive, becoming so overwhelmed that she slaps him across the face. In Ten-Forward, tempers flare over seats at a table, triggering a massive brawl that involves everyone in the room.
Deanna Troi and Dr. Crusher soon notice that these strange events coincide perfectly with Ambassador Sarek's arrival, prompting both to theorize that he may be suffering from a very rare form of degenerative disease called Bendii syndrome, which only affects Vulcans over the age of 200. Similar to Alzheimer's Disease in humans, Bendii Syndrome corrodes the Vulcan's disciplined control of their emotions until they spill out and overwhelm the victim.
The syndrome has a particularly unfortunate side effect: the Vulcan mind emits "broadcast empathy" which can destabilize the emotions of other beings in the vicinity at random, causing violent outbursts. Concerned about the upcoming conference with the Legarans, Picard nevertheless hesitates confronting Sarek about the matter. Commander Riker expresses his annoyance, and the two engage in a screaming match on board the bridge before Commander Data is able to calm them both down. Realizing what is happening, Picard tries to relax, and finally agrees with Riker.
Picard brings these concerns to one of Sarek's aids, who flatly denies the allegations whilst subtly threatening Picard's career if he pursues the matter. Picard asks Commander Data to speak with Sarek's second aid, Sakkath, a young Vulcan with whom the two share a mutual respect. Data is able to successfully extract the Vulcan's true concerns. The conference is in danger. Sakkath has been channeling his own strength onto Sarek in an attempt to bolster his emotional control, but admits that the stress of the upcoming conference has overwhelmed even his natural talents.
Assured of the situation, Picard confronts Sarek personally regarding the issue. At first, Sarek greets Picard freely and encourages him to ask whatever questions he wishes, even going so far as to volunteer for the Bendii Syndrome test. When Picard states that the results will not be available until days after the upcoming conference, Sarek begins to feel backed into a corner. When Sarek continues to refute the allegations, Picard openly exposes Sakkath's assistance in strengthening Sarek's emotions.
When Sakkath confirms the revelation, Sarek is infuriated. He and Picard are left alone in Sarek's quarters to talk. Sarek tries hard to convince Picard that he is in full control of his faculties, but Picard confronts the Ambassador about his weeping during the concert the previous night. Sarek becomes more and more agitated until he finally loses control, his body in spasms as he hollers in anger.
Picard is ready to bring the conference to a close, despite the Legaran's refusal to mediate with anyone else. Perrin visits Picard's ready room to beg his reconsideration. Realizing the dangers of letting an unstable Vulcan mediate a trade negotiation, Picard is forced to again refuse. To compromise, Perrin suggests a mind-meld with Sarek to stabilize his emotions long enough for the conference to finish. Picard volunteers freely, despite Sarek's warning that such a connection could have disastrous consequences for the human mind. Realizing Picard's honorable willingness to make these negotiations work, Sarek agrees.
After the meld, Sarek is once again in full control of his emotions and stability, even taking on some of Picard's personality quirks in the process. While he begins the negotiations with the Legarans, Picard retreats to his quarters where he suffers through a severe and terrible onslaught of Sarek's most negative emotions. Dr. Crusher stays on hand to monitor Picard's elevated blood pressure, heart rate and brain stimulus which are now in a state of extreme excitement.
Picard is forced to endure the terrible whirlwind of Sarek's darkest sorrows, regrets and anger. Through Sarek's eyes, he calls out to his son, Spock, and his first and second wives Amanda and Perrin, apologizing for not being able to show them the love and the tenderness that he wanted to, all of which is shunned by Vulcan society. Dr. Crusher manages to momentarily snap Picard out of the dark emotions, after which he is able to describe just how overwhelming Sarek's regret and sadness truly is. He quickly loses control again as another wave of emotion comes crashing down, forcing him to sob uncontrollably in Crusher's arms.
Afterward, Sarek prepares to take his leave. Picard awaits his arrival with Perrin in the transporter room where he is able to express Sarek's immense love for her in a way the Ambassador could never do. Perrin reassures Picard that she has been completely aware of that love since the very beginning. When Sarek arrives, he thanks Picard for his gesture of kindness, telling the Captain that they will both retain the best parts of each other. Picard suggests that he is definitely getting the better part of that bargain. Sarek departs, leaving Picard fully aware that the Bendii Syndrome will, unfortunately, only get worse.