What really happened before Leonard Nimoy's Spock traveled back in time for J. J. Abrams' 2009 Star Trek movie, and what was the Vulcan's true connection to Eric Bana's villainous Nero? When rebooting the Star Trek movie franchise, Abrams sought to connect his new series to the franchise's history, but also wanted to break free from around half a century's worth of continuity. The director's solution was time travel. The Spock of the original Star Trek series goes back into the past alongside a small band of angry Romulans and drastically changes history, inadvertently creating a new reality dubbed the Kelvin Timeline.
Predictably, the time mechanics of the 2009 Star Trek movie create some confusion, and this is perhaps partly because the film only shows the tip of the iceberg. The rest of Spock's journey and the primary motivation for Nero's hatred is explored elsewhere, most notably in the Countdown graphic novel by the movie's writers, Roberto Orci and Alex Kurtzman. On screen, Leonard Nimoy explains to Kirk that he comes from 129 years in the future, a time threatened by the existence of an exploding supernova that could erase the entire galaxy. The shockwaves destroyed Romulus first, but Spock managed to prevent further damage by using red matter. Unfortunately, this wasn't enough for a group of Romulan miners lead by Nero, and the resulting battle dragged Spock's Jellyfish ship and Nero's Narada into the black hole the Vulcan had created.
The full story sheds more light on the matter. Prior to the emergence of the supernova, Spock was acting as the Federation's ambassador to Romulus, despite some reservations about a Vulcan holding this role. After learning of the impending cataclysm, Spock suggested his red matter plan to the Romulan senate, but his concerns were dismissed as over-zealous and paranoid. Trusting his own scientific mind, Spock knew that Romulus was in grave danger and teamed up with one of planet's miners, Nero, to carry out his plan despite the senate's decision.
Having witnessed the power of the supernova while on a mining expedition, Nero is gravely concerned about his planet's future, and Spock promises to do all he can to protect Romulus. Nero leaves his wife and child in order to aid Spock's red matter mission. The pair travel to Vulcan where, unsurprisingly, the council also oppose Spock's plan, and Nero threatens to hold each member responsible should Romulus fall. Fortunately, a certain Jean-Luc Picard is the Federation's ambassador to Vulcan and he opts to aid Spock and Nero's quest, enlisting Geordi La Forge to craft Spock's Jellyfish ship, as seen on the big screen. Ultimately, of course, Spock acts too late to save Romulus, Nero attacks him in anger, and both are sent back in time.
This full version of Spock's time travel and Nero's backstory serves several purposes. Firstly, Eric Bana's Nero becomes a far more rounded villain. The Countdown backstory explains why Nero is so bitter towards the Federation and gives him an extra motivation to destroy Vulcan, punishing those who hindered the potential salvation of his home planet, rather than simply to make Spock feel his pain. Seeing Nero as a working family man before all the facial tattoos and wanton violence also adds much depth to his character and establishes a more personal connection with Spock. The two had a deal, promises were made and, as far as Nero is concerned, they were not kept.
The prequel story also helps tie J. J. Abrams' Kelvin Timeline into the Prime Timeline by introducing more connections than just Leonard Nimoy's Spock. The revelation that a character from Star Trek: The Next Generation built the Jellyfish, for example. Moreover, Picard is likely sharing in Spock's shame and guilt at not having been able to stop the destruction of Romulus. This may well feed into the forthcoming Star Trek: Picard series, in which Romulans appear to play a significant role. Tellingly, Countdown writer, Alex Kurtzman, is acting as showrunner for Patrick Stewart's upcoming series.
Another key addition to the story of the 2009 Star Trek movie is that Spock knew full well his mission to stop the singularity was a one-way trip. It's suggested in the film that Spock was essentially dragged into the past by Nero. This is only half true, and the comic story makes it clear that Spock had no intention of escaping, making his sacrifice all the more heroic.