Shinzon was the chief antangonist in the movie Star Trek: Nemesis, in which he led a coup that overthrew the Romulan Senate, and then attempted to destroy Earth, the center of the United Federation of Planets. In the world of Star Trek, this happened in 2379.
Shinzon was a clone of Captain Jean-Luc Picard from Star Trek: The Next Generation. Shinzon was created by the Romulans with the intent of replacing Picard, and using the clone to bring about the downfall of the Federation.
With the successful Federation-Romulan alliance in the Dominion War, the plan was abandoned, and Shinzon was sent to work the mines on Remus, the twin planet of Romulus.
In 2379 the Romulan Senate was briefly overthrown in a Reman coup led by Shinzon. The Praetor and the Romulan Senate were killed by thalaron radiation which emanated from a handheld device. Shinzon became Praetor and the Remans took over the Romulan Senate. However, the cloning process that had created Shinzon was killing him, and he required a fresh infusion of DNA from Captain Picard to stay alive.
Shinzon decided to fulfill his destiny by building a new starship, the Scimitar, which was the most powerful and fast of its time. It also carried a new type of cloaking device which rendered it completely undetectable to current technology. In this vessel, Shinzon and the Remans met Captain Picard and his crew from the Enterprise during a diplomatic mission to Romulus.
Shinzon would later meet his death aboard the Scimitar as it was destroyed by the Enterprise and two Romulan warbirds.