So, I imagine most people think that the reason Spock Prime has refused to give Starfleet future information is some sort of noble variant of the temporal Prime Directive. Nope. He's old, he's grown comfortable with his emotions, he's frustrated by his failures with reunification and with trying to save Romulus and with Nero and the destruction of nuVulcan. He's had enough.
So he has taken his 10,000 Vulcan refugees, and gathered everything he knows about - future Starfleet tech, the Doomsday Machine, Korby's android making device, Data's head, so on and so forth, shutting many of them down as the threats that some of them may have been in his home timeline in the process - and now, from his secret base in an undisclosed location, he is running a massive operation to put right what he thinks has gone wrong and just generally make himself happy. Starting with the conquest of Romulus by the Vulcan refugees.
The weakness in his plan? He invited young Spock and crew to join him in his endeavor. So they're trying to stop his plans from being carried out. But every time they do, and they do a few times, he uses The Guardian of Forever (that's where his base is) to come out on top, anyway.
Will the crew manage to stop old Spock? Do we really want them to?!
Rated NC-17, Runtime: 27 hours, 30 minutes, Rotten Tomatoes rating: their server melted.