This is one of the best conceived and executed Star Trek plots by Jerome Bixby, the only science fiction writer to remain with the series from season 2 to season 3 of TOS. 38 Federation vs 38 Kilngon crew are driven to fight each other under the control of an alien being. The viewer sees many bizarre scenes: Chekov believes that the Klingons killed his older brother Piotr and wants to avenge Piotr's death. But, in reality, Chekov was an only child, as Sulu tells Kirk. Scotty and McCoy are both filled with feelings of hate and racism towards the Klingons and Scotty criticisesSpock's Vulcan heritage. The real Scotty and McCoy would be trying to reason with Kang and reach a truce with their unwanted Klingon guests. Bixby cleverly shows how the alien entity was influencing and controlling people's thoughts and emotions early in the episode.
This is one of the top 6 episodes of season 3 and I rate it highly for its excellent execution and believability. Both Kirk and Spock recognise the true enemy is the alien which feeds on feelings of mutual hate and conflict, rather than the Klingons who have seized control of parts of his ship. The part about intraship beaming was a novel method for Kirk to quickly reach Kang and arrange a mutual truce to weaken and expel the entity before the Enterprise's dilitium crystals are fully depleted. Michael Ansara is superbly cast in his role as the Klingon commander Kang who has no reservations about torturing Chekov or turning off the life systems in those sections of the Enterprise which the Federation crew still control. Bixby also assigns a meaningfulrole to Mara, Kang's wife, a scientist in her own right, and one of the only Klingon women ever depicted in TOS. Shefinally convinces Kang to reach a truce with Kirk and expel the alien. Bixby's script is excellent; its just a pity most season 3 TOS episodes weren't this well written or made.