The plot by the Zhat Vash, a secret Romulan society, to wipe out synthetic life, behind the Mars attack and exposed during the Coppelius incident of 2399.
The Zhat Vash were an ancient and secret Romulan society, more hidden even than the Tal Shiar, devoted to stopping the creation of synthetic life. Their fear came from the Admonition, a warning left long ago by synthetic beings, which showed the destruction that artificial life would bring on organic civilizations.
The group's agents reached deep into the Federation. Commodore Oh, the head of Starfleet Security, was secretly one of them, and the Zhat Vash were behind the 2385 attack on Mars, carried out by subverted synthetic workers. The attack led directly to the Federation's ban on synthetic life.
In 2399 the Zhat Vash learned that Data's legacy had produced new synthetic beings, including the twin sisters Dahj and Soji. The siblings Narek and Narissa hunted Soji, and Oh brought a Romulan fleet to destroy the synthetic colony on Coppelius. The plot failed when Soji shut down the synthetic beacon and a Starfleet fleet arrived.