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Leonard Nimoy -- who had only returned as Spock for Wrath Of Khan on the understanding his character would be killed -- clearly had second-thoughts after seeing the finished product (which included a get-out clause to make Spock's resurrection possible, fortunately.) Star Trek II was a huge success at the box-office and encouraged Paramount to push ahead with a third film, with Nimoy himself installed as director, working from a script by Khan storywriter Harve Bennett.
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Fortunately, Spock's saturnine father, Ambassador Sarek (Mark Lenard), arrives and explains McCoy's strange behaviour -- giving Kirk hope that his friend's body and soul can be united in a Vulcan ceremony. But, time is of the essence; so, with the help of his always loyal crew, Kirk smuggles "mad" McCoy out of Federation custody and steals the Enterprise to fulfill their mission. But, lurking in the depths of space as an added complication is Commander Kruge (Christopher Lloyd), a Klingon who's learned of the Federation's Genesis experiment and, in an unfortunate echo of Khan, aims to steal the technology and use it as a weapon.
Star Trek III doesn't deserve its poor reputation, really, although it's by-the-numbers moviemaking that doesn't hit too many highs. The main issue is that it's designed as a way to renege on Spock's death, thus giving audiences a sense of frustration that this isn't a brand new adventure. Certainly, beyond some fun moments with a "possessed" McCoy and the exciting capture of the Enterprise (including the comical sabotage of Excelsior), there's not much to grab audiences until the third Act, when events all come to a head on the disintegrating Genesis planet...
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So, while by no means a great entry into the Trek pantheon, it's not as awful as you probably remember it being. This was actually the film where the crew started to feel a fun, likeable group (as Trek II was primarily focused on Kirk/Khan rivalry), and it successfully builds to a strong finish that helps dilute the plodding first half. Mind you, it's always confused me that Kirk essentially spends the whole film acting in defiance of Spock's mantra ("the needs of the many outweigh the needs of the one, or the few") by risking everyone's lives to get Spock back!
Paramount Pictures
105 minutes
Director: Leonard Nimoy
Writer: Harve Bennett
Cast: William Shatner (Admiral James T. Kirk), Leonard Nimoy (Spock), DeForest Kelly (Dr. Leonard "Bones" McCoy), George Takei (Hikaru Sulu), Walter Koenig (Pavel Chekov), Nichelle Nichols (Uhura), James Doohan (Montgomery "Scotty" Scott), Bibi Besch (Carol Marcus), Merritt Butrick (David Marcus), Mark Lenard (Ambassador Spock), Judith Anderson (High Priestess T'Lar), Robin Curtis (Lt. Saavik), Christopher Lloyd (Commander Kruge), Robert Hooks (Fleet Admiral Morrow), James Sikking (Capt. Styles) & John Larroquette (Maltz)