Famed Vulcan Ambassador Sarek (Mark Lenard) comes aboard the Enterprise to conduct delicate negotiations with the Legarans, an alien species that Sarek has single-handedly been able to open relations with on behalf of the Federation. Sarek intends to seal the talks as the crowning achievement in his storied career. While en route to meet the Legarans, however, odd occurrences of flaring tempers begins to affect members of the crew, with escalating urgency. Meanwhile, it becomes evident that Sarek himself may be having problems controlling his emotions; Picard sees a tear in the Vulcan's eye during a concert in Ten-Forward.
The flaring tempers begin ominously but harmlessly, as Wesley and Geordi get into a shouting match over who's more hopeless when it comes to women (ah, a perfectly appropriate nerd fight!), and slowly escalates: Crusher slaps her son in the face for no good reason, and ultimately an entire bar brawl breaks out in Ten-Forward — a visual that proves as amusing as it does odd. What's going on here? Crusher believes that it's a case of a rare Vulcan mental illness that results in a loss of emotional control. The side effects are unintentionally inflicted upon others, caused by Sarek's telepathic abilities randomly projecting emotions, and thus havoc, on members of the crew. Picard runs into resistance with Sarek's staff and wife Perrin (Joanna Miles) when he recommends that they delay the negotiations. A delay would derail the talks completely, and Sarek will not hear of it.
In addition to being a rare, direct, fan-welcome bridging of TNG and TOS, "Sarek" is an obvious example of the "actor's episode." Some Trek outings highlight action or visual effects as their main selling points; this one highlights performances. It's a good, solid story that's elevated by two critical acting scenes. One is where Picard confronts Sarek, and Sarek attempts to prove his competence while his emotional control is not being held together by his aide Sakkath (Rocco Sisto). Sarek's gradual deterioration as depicted by Mark Lenard in this scene is an explosive (and heartbreaking) sight to behold. The other big scene comes when Picard offers to accept a mind meld that temporarily allows Sarek to regain control of his emotions long enough to finish the negotiations. In the meantime, Picard's mind must host Sarek's savagely intense and unfiltered Vulcan emotions. Patrick Stewart is completely uninhibited in showing a rambling, anguished explosion of Sarek's inner voices, fury, and soul. It's a remarkably brave performance that makes us believe in this intriguing premise.
Thematically, the story provides a subtle allegory on the elderly and the mentally ill, regarding the issues of humiliation they must endure when the circumstances of their health force them to abandon important parts of their lives and identity. There is no cure for Sarek's condition; like Alzheimer's, it will slowly continue to steal him away, separating the mental faculties from the man.