Sometimes a Great Notion is an episode of the TV series, Battlestar Galactica, and aired during the show's fourth season. The episode continues from "Revelations", where both the colonial fleet and the rebel Cylons finally found Earth, only to find it devastated by a nuclear holocaust at least 2,000 years before, and the discovery leads to despair all across the fleet. The episode was not given an opening title sequence, but the survivor count for the episode was 39,651
Both the Human fleet and the rebel Cylons are disillusioned after finding Earth devastated by a nuclear holocaust, which occurred at least 2,000 years prior to the events of the episode. After research on the bodies found in the soil, the Cylons and Dr. Baltar conclude that the remains are not Human, but Cylon in origin. The rebel Cylons further state that the mechanical Cylon remains resemble older Centurions of theirs but are of a kind unknown to them. Following this, they speculate that the Thirteenth Tribe consisted of a different kind of Cylon which existed over 2,000 years ago and moved out from Kobol to populate Earth. When addressing the fleet, Admiral Adama never states this fact, leaving the fleet in the belief that the Thirteenth Tribe was human. Meanwhile, Samuel Anders, Galen Tyrol and Tory Foster receive short memories of their lives on Earth 2,000 years ago. Kara Thrace has Leoben help track the origins of the beacon. However, what they find is a crashed viper, with a corpse with Starbuck's dog tags. After Kara recites what the Hybrid has told her, Leoben becomes scared and retreats, with Thrace wondering what she is. In shock, Thrace takes the body and burns it on a pyre, and decides to not tell anyone about what she found, making people believe she lost the signal.
In the fleet, President Roslin burns her book of Pythia, and is unable to address the fleet because of feeling that she led the fleet nowhere, and believing the prophecy is a lost cause. Lee Adama and Anastasia Dualla revisit their relationship, and after their evening together, Dualla returns to her quarters, and commits suicide. Devastated by this, Admiral Adama acquires a handgun from a Marine and attempts to coerce Tigh into killing him, which fails. Adama breaks down relieved and converses with Tigh about past memories, and Tigh reminding Adama of his duties as commander of the fleet. Adama eventually steps back into the CIC, and makes an announcement that he will find a home for the fleet, gives instructions to search for any nearby habitable star systems, and invites their new Cylon allies to join them. As the fleet prepares to leave Earth, D'Anna, devastated by the knowledge that history repeats itself endlessly, decides to remain on Earth to die, rather than being hunted by Cavil. Tigh walks into the sea and receives a flashback of his life on Earth. He realizes that his wife, Ellen, is the last of the Final Five Cylons. In the vision, a dying Ellen tells Tigh that "everything is in place" and that they will be "reborn, again, together". They are destroyed in this vision by a nuclear blast.