Then, not long after he comes on board, tempers start flaring up on board the ship: Wesley and Geordi start arguing with each other heatedly while setting up the conference room and are broken up by Riker. Then, after a Mozart recital which Sarek attends (and at which Picard actually sees Sarek cry), tempers get worse: Bev slaps Wesley for no reason, and Ten-Forward becomes the site of a bar brawl.
Bev and Troi eventually advance a theory: Sarek himself may be the cause, due to a very rare malady contracted by aged Vulcans known as Bendai's syndrome, in which all emotional control is gradually eroded away. Eventually, Data confronts Sarek's young aide Sakkath about his odd behavior (checking to see how Picard's diplomatic skills are, and so forth), and eventually finds that Sarek indeed suffers from the disease, and that Sakkath has been trying to help Sarek's control, with limited success. This will jeopardize the treaty with the Legarans (since Sarek's the only one they'll speak to), but Picard is forced to convince Sarek of this anyway, over the objections of both Perrin and Sarek's chief of staff, Mendrossen (also a human).
Picard manages to convince Sarek of his ailment (by driving him virtually incoherent with rage, which he shouldn't be able to do), but is then struck by remorse over what he has done to such a great man. Upon Perrin's later suggestion, Picard suggests to Sarek that the two of them mind-meld, giving Sarek the added control he needs, and letting Picard serve as a temporary well for Sarek's emotions. Despite the risk, they go through with it, and while Picard/Sarek rages over his inability to tell people he loves how much he loves (or loved, in the case of the dead) then, Sarek/Picard manages to conclude the treaty. Sarek expresses his gratitude to Picard, and says they will each retain the best part of the other. He leaves, his mission successful, but his control eroding further...and Bendai's Syndrome has, at present, no cure.