Benjamin's Flight is the DLC and prequel to Thomas Was Alone. It talks about how the splitters became and Sarah's past.
Once every cycle Benjamin and his father made a climb, where is unsure. Benjamin had seen a light, the fountain of wisdom, and always wanted to go there. One day, Benjamin's father builds a jet-pack, which Benjamin then steals. His intention is to fly to the fountain, touch it, and fly home again. On his way he meets Anna. Anna had previously tried to reach the fountain, but mentions "they" had seen her, and she bailed. She warns Benjamin about the dangers of he fountain, of the blinding light and the pixel clouds that protected it. Benjamin ignores her and says he can just fly above the clouds. As Anna keeps trying to get Benjamin to go back, they encounter another AI, Sarah. Sarah, unlike Anna, is very impressed by Benjamin's jet-pack, and Benjamin himself. She encourages Benjamin and considers herself his "disciple". Benjamin is pleased by her admirations. Eventually Anna gives up, says she's done with adventuring, and locks herself away. Benjamin and Sarah fly on ahead towards the fountain, but has to leave Sarah before going to the fountain. When he gets there, nothing else mattered. But when he touches the fountain, the immense light from it blinds him, and the pixel clouds are released. The next level Benjamin says he'll rest, and try to find his way home the next day. After that, Sarah is seen falling, talking about how the fountain ruined everything, and how she'd never see Benjamin again, before falling into a room similar to the one Sarah is first met in in the main game.