A combination of two stupid mistakes led to the circumstance of Kirstie Alley not reprising her role as Saavik in this film, and being replaced by the frankly poorer performance of Robin Curtis. Firstly, whoever at Paramount drew up her initial contract for The Wrath of Khan neglected to put in a sequel clause with a fixed salary - something which was standard for new actors at the time. This meant they couldn't cast Alley for a pre-agreed fee, and opened the sequel appearance up to negotiation.

The second stupid thing was that Kirstie Alley, Leonard Nimoy, and producer Harve Bennett were all eager for her to reprise the role of Saavik. Kirstie had been a huge Star Trek fan since childhood, and Nimoy and Bennett were both impressed by her performance. They discussed a salary figure, and there was general agreement. Then Alley's agent stepped in. The agent demanded a "huge" salary increase, which Nimoy and Bennett felt unable to accept, as it was actually higher than DeForest Kelley's! Unfortunately it also seems the agent kept Alley in the dark about this negotiation, since she later expressed surprise that the negotiation had broken down.

So, I don't know who you are, Kirstie Alley's agent from 1982, but you have a lot to answer for, and I hope she fired you soon afterwards.

With that off my chest... The Search for Spock is actually the best of the infamous "odd-numbered bad Star Trek movies". There are several great scenes in it, and the overall story is generally not bad.

Transcript

1
Captain's log, Stardate 8210.3. Returning to Earth after Spock's death. Saavik and David have stayed to explore the Genesis planet.
Sulu: Admiral! Someone broke into Spock's quarters.
2
{Spock's quarters}
Kirk: I'll go in before security.
Spock-like silhouette: Jim... you left me behind on Genesis. Take me home!
3
{it's McCoy!}
Kirk: Bones!
McCoy: Remember!
Kirk: Remember? Remember what?
4
Caption: Earth:
Kirk: Ambassador Sarek!
Sarek: Spock entrusted his katra, his living spirit, to you!
Kirk: What?
Sarek: Clearly I need to mind meld to get through your thick skull.
5
Sarek: {mind melding} My apologies. His katra is not here. My son is truly dead.
Kirk: Huh.
Sarek: I guess he never really liked you much after all.
Kirk: Noooo!!!
6
{viewing footage of Spock and McCoy}
Spock: {on screen} Remember.
Kirk: He left his katra in Dr McCoy!
Sarek: Get him and Spock's ass to Mars... I mean, Vulcan.
7
{a bar}
Kirk: Give me Enterprise and let me go to Genesis.
Admiral Morrow: No. And when I say no to the most regulation-bucking captain in Starfleet history, I have no expectation that you'll defy orders and do it anyway.
8
Sulu: I can find us a charter ship to Genesis.
Kirk: Why break one rule when you can break a dozen? Let's steal Enterprise!
9
Caption: Meanwhile, on Genesis:
Saavik: That life form reading. It's Spock, regenerated as a boy!
David: Oh yeah. Genesis will do that. Did I forget to mention?
10
{a Klingon ship in orbit about Genesis}
SFX: Kaboom! {this ship destroys USS Grissom}
Kruge: Great Scott! I wanted prisoners so I can get Genesis!
Torg: There are some on the planet!
11
{on Genesis}
Saavik: Spock is ageing. Soon it will be the pon-farr. He must mate, or die.
David: I'll... just leave you two alone.
Caption: Kinky Vulcan finger sex
12
Kruge: Give me Genesis!
David: Is this a holdup?
Kruge: It's a science experiment!