Picard is revealed to be Shinzon from the future, extracted at the moment of his death by the 29th century Section 31 and the secretly rehabilitated Captain Braxton. His mission is to get his twin married to Doctor Crusher in order to fix the timeline, because after Picard's time incursion in All Good Things they never married, and this resulted in the loss of the Spherebuilders', the Temporal Cold War and the war against the Red Angel, because their great granddaughter Louise-Estelle Picard was instrumental in winning all three of them.
After installing Federation-friendly puppets in the governments of Talos IV and the homeworld of Species 8472, Section 31 have taken their simulation technology, and built a giant holodeck of the whole galaxy. The entire Picard series is revealed to be taking place on that holodeck, where Shinzon/Picard is running different scenarios for the timeline, and is causing more catastrophic timelines with each season.
He finally goes rogue and decides to run the experiments on the real timeline. He can't really travel back in time, because he's – like Moriarty – stuck on the holodeck, and incompatible with pre-26 century holoemitters, so he has to run everything from the holodeck and only communicate through time. That's when he is revealed to be Future Guy.
After installing Federation-friendly puppets in the governments of Talos IV and the homeworld of Species 8472, Section 31 have taken their simulation technology, and built a giant holodeck of the whole galaxy. The entire Picard series is revealed to be taking place on that holodeck, where Shinzon/Picard is running different scenarios for the timeline, and is causing more catastrophic timelines with each season.
He finally goes rogue and decides to run the experiments on the real timeline. He can't really travel back in time, because he's – like Moriarty – stuck on the holodeck, and incompatible with pre-26 century holoemitters, so he has to run everything from the holodeck and only communicate through time. That's when he is revealed to be Future Guy.