Jean-Luc Picard and Spock once met during the events of Star Trek: The Next Generation - so what happened when the legendary Vulcan took on the titular hero of Star Trek: Picard? In the new show's third episode, "The End Is The Beginning," sharp-eared Star Trek fans likely heard Cristobal Rios' Emergency Navigational Hologram mention that Picard had worked with the legendary Spock in the past, along with a laundry list of other prestigious accomplishments. There's little doubt Picard himself would list his encounter with Spock near the top of his achievements.
Thirty years before the events of Star Trek: Picard - and a century after the events of Star Trek: The Original Series - Picard and the Enterprise were given a top secret mission to recover Ambassador Spock, whom Starfleet feared had defected to the Romulans, in the classic TNG two-parter "Unification." This was something of a personally complicated mission for Picard, as he had become close to Spock's father Sarek on a previous mission, and he had the unhappy duty of informing Spock of Sarek's passing.
Of course, Spock hadn't defected to the Romulans; he had instead disappeared into the planet's burgeoning underground which sought reunification between the Romulan and Vulcan races. After foiling a plot by the Romulan officer Sela to conquer Vulcan, Spock chose to stay on Romulus in hope of fostering the reunification movement further. Before he departed, Picard offered to share his previous mindmeld with Sarek with Spock, so that the son could know once and for all how his father truly felt about him, as they had never chosen to meld. Spock was momentarily overcome with rare emotion as he realized how much his father loved and respected him.
Spock and Picard would go on to defend Romulus in different ways over the next two decades. Picard would spearhead the rescue mission which attempted to save as many Romulan lives as possible before the planet's sun went supernova, though that mission would be derailed by the synth attack on Mars that wiped out the Romulan rescue fleet. Despite Picard's loud protestations - and eventual resignation - the Romulans were left to fend for themselves, and Romulus itself was indeed destroyed with millions of inhabitants.
Using a vessel armed with technology developed by the Vulcan Science Academy, Spock resolved to save Romulus by shunting the supernova into a black hole. He was too late to save Romulus, but he almost certainly saved millions of lives. Spock himself was sucked into the black hole and ended up in the relative past of an alternate universe, known as the Kelvin timeline, which was chronicled in the Chris Pine-starring film trilogy.
Picard and Spock are unlikely to cross paths again in Star Trek: Picard, as Spock died of old age in the Kelvin timeline, having never returned home. But while the two men only had a fleeting relationship, they're forever bonded in their efforts to save the Romulan people from forces without and within, and through their bond with Sarek.