An attack in 2269 by the disembodied survivors of the planet Zetar on the Federation library planetoid Memory Alpha, which killed its staff and wiped its archive.
Memory Alpha was a new and unshielded central library holding the knowledge of every Federation member. The USS Enterprise was carrying Lieutenant Mira Romaine there to supervise the installation of equipment when a storm-like cloud of lights passed through the ship and then struck the planetoid.
In the Original Series episode The Lights of Zetar the landing party finds the technicians dead or dying of brain damage and the memory core burnt out. The lights are the combined life force of the last hundred Zetarians, seeking a body in which to live again, and Romaine's mind proves compatible.
They take possession of her. Scott, who has fallen in love with her, places her in a pressure chamber, and the rising pressure kills the Zetarians and frees her. She goes on to help rebuild the library.