In the late 2240s or early 2250s, Simon served under Commodore Robert April as first officer of the ISS Constellation. She and Commodore April had served together for a long time, and April permitted her to be more informal with him than other officers under his command. She served on the mission to capture a Klingon weapon capable of destroying the atmospheres of class M planets. Simon was stunned by what the weapon did to Donatu V and helped April capture a Klingon scientist on that planet.
At the conclusion of the mission, April's wife Sarah April threatened to use Simon as a guinea pig for her experiments unless the Commodore acquiesced to her demands. (TOS - Mirror Universe short story: "Ill Winds")