The secret extermination of the Regressives, an Enaran minority who rejected modern technology, which later generations were taught had been a peaceful resettlement followed by a natural dying out.
In the Voyager episode Remember, the ship carries a group of telepathic Enarans home. B'Elanna Torres begins having vivid dreams in which she is Korenna Mirell, a young Enaran woman in love with a Regressive named Dathan Alaris.
The dreams show how the Regressives were first restricted, then rounded up for what was called resettlement on a distant colony. In truth they were killed. Korenna, pressed by her father Jareth, betrayed Dathan and watched him executed, then spent her life teaching the official story.
The dreams are being sent by an elderly passenger, Jora Mirell, who is Korenna herself and wants the truth to outlive her. She dies before the journey ends. The other Enarans deny everything and Janeway will not force the issue, but Torres shares the memories with a young Enaran, Jessen, who agrees to receive them.