Illogical Mission

After the events in "Mirror, Mirror", mirror Spock tries to follow the vision of the Federation he saw in our universe's Dr. McCoy by asking mirror Captain James T. Kirk not to exterminate the Halkans. He realizes he has no hope of reforming Kirk and later strangles him to death in his quarters while Marlena Moreau watches. Marlena demonstrates how to use the Tantalus field to Spock on Kirk's body. Later, Spock listens to his human desires and keeps Marlena as the captain's woman, and then performs a mind meld on her. He begins plotting a revolution to destroy the Terran Empire and raise a new galactic order. 
Before the conference at Babel, Spock welcomes Sarek, whom he has not talked to in eighteen years, and the Vulcan delegates aboard the Enterprise and gives them a tour. He has a private conversation with Sarek in which Spock reveals his plan. He knows the Empire's fall is inevitable, but wants to preserve knowledge and to save its people by giving them freedom first. Spock will seize the throne and replace the Empire with a republic. He also reveals that Marlena is now his fiancée. 
Spock starts off his grand scheme by negotiating a treaty between the rival Elasians and Troyians. The Empire had seen huge deposits of dilithium crystals on both planets, but the planets were too well armed to be conquered, so they provoked a war to weaken the two races to conquer them when they had weakened each other. Spock convinces the Dohlman of Elas to wed the Caliph of Troyius and unite the two worlds to build up their economic power against the Empire. Elaan also grants Spock, not the Empire, exclusive mining rights of dilithium on her planet, guaranteeing Spock an admiral's position in Starfleet. 
Meanwhile, Montgomery Scott, Leonard McCoy and Nyota Uhura are discussing Spock's strange behavior ever since the Halkan mission. A while earlier, the Enterprise ran into a Klingon cruiser, but when a Starfleet boarding party beamed aboard, there were no Klingons aboard and it looked like they had just disappeared. The three of them suspect that Spock has Vulcan psionic powers, and Uhura resolves to kill him, but she is suddenly vaporized by the Tantalus field. Rumors continue to spread throughout the ship and the fleet, exactly as Spock plans. 
When word reaches Empress Hoshi Sato III, she decides to make Spock an admiral to gain his loyalty. 
Admiral Spock orders his first officer of the ISS Enterprise, Commander Willard Decker, to join him, Ensign Saavik, Lieutenants Xon, Solok and Stang at a Starfleet Admiralty conference on Deneva. The landing party goes to the security room, where Stang reports that there are no other admirals at the conference. Spock tells a dumbfounded Decker that he knows this is a trap. When the Vulcans are not looking, Decker tries to fire his phaser at Spock, but is stopped by the phaser-dampening field in the room. Saavik and Xon stab him and slit his throat. 
Grand Admiral Matthew Decker, Will's father, is furious when he hears the news that his son failed in their assassination attempt and is now dead. His first officer, Commander Hiromi Takeshewada, also informs him that he must contact the Empress as soon as possible. Sato orders him to kill Spock because they know he will seize control of Starfleet soon, and because of his popularity within Starfleet. He sends a message to the rest of the fleet loyal to the Empress telling them to assemble at the Terra Nova system and destroy the Enterprise. 
The Enterprise intercepts the encrypted message from Decker and moves to Terra Nova to "surrender". The ship arrives with shields and weapons powered down, and Spock offers to beam to Decker's ship, the ISS Constellation, to formally surrender. Once he arrives on the Constellation's bridge, the crew mutinies against Decker. Takeshewada, whom Spock had recruited for his cause years ago, relieves and kills Decker. Crews on all other ships throughout Starfleet have also mutinied, and Spock now has control of 61.3% of the ships, while the rest are captained by neutral officers. The Empress watches as Spock assumes the rank of grand admiral, and orders him to go to her palace next week so she can congratulate him. 
Right before the reception at the palace, Spock orders Saavik to his quarters. He asks for her thoughts about what he is about to do and decides she is worthy. He shows her how to use the Tantalus field and leaves the fate of the revolution in her hands. She has five minutes to decide whether to let the Imperial guards kill Spock and try to gain power for herself later, assassinate Spock with the field and gain favor with the Empress, or help him towards the future of the Federation he shows her in a mind meld.
Saavik realizes that she couldn't support Spock in his illogical mission to stage a coup. So she uses the Tantalus Field to kill Spock. Marlena Moreau, Spock's wife, watches from the shadows. Marlena then takes out her phaser and shoots Saavik, killing her! However, the intervention came too late to save her husband.