She was the daughter of famed United Federation of Planets Ambassador Robert Fox and was born on the planet Earth. By the time she was 19, she had left her parents home due to a personal dispute she had with her father. After this chain of events, she left to join a revolutionary organization because of the fact that she hated him. At some point when she reached the age of 24, she was kidnapped by the Orion Victory League who brainwashed her into an assassin and taught the Cellular metamorphosis technique from Antos IV allowing her to change her form.
Given the code name Enigma, she was dispatched in the 2280s on a mission to disrupt the peace process between the Federation and the Klingon Empire being held on Babel which was being led by her father; Robert Fox. Her mission led her to journey onto the USS Enterprise under the command of Captain James T. Kirk where she disguised herself as an Andorian Starfleet officer. She later transformed into Lieutenant Richardson who was part of the security detail for the ambassador allowing "Enigma" access to his quarters. Once there, she transformed into a multitentacled Rigellian decapod and attacked the Ambassador but was driven off by Captain Kirk along with his crew. Though "Enigma" managed to escape, she left a tape behind for Ambassador Fox with a message from the Orion Victory League that announced his death by the hands of his own daughter who was revealed to be the assassin Enigma.
She later attacked Chief Engineer Montgomery Scott during her guise as a Dimorian water rat who's venom killed its prey in 24 hours. After hiding Scott, she took on his form and was present at the Transporter Room during the beam down of Ambassador Fox. However, her identity was revealed when Captain Kirk mentioned "Queen to Queen's level 3" which was the phrase he had mentioned the previous time when they discovered the existence of a shapeshifter. As phaser's were aimed at her, Trisha transformed into a bird and flew into the transporter where she was beamed onto Babel.
During the conference, Trisha sensed a hole in the security detail and attacked by transforming herself into a Gorn. However, this was a trap set by Captain Kirk but before they could spring it on Enigma, she transformed herself into a duplicate Kirk leading to confusion over which was the real one. But Doctor Leonard McCoy stated that he would inject Retinax V into both of them which was a drug that the real Kirk was allergic to. Learning of this, Trisha attempted to escape but was incapacitated by Saavik who used a Vulcan nerve pinch on the brainwashed assassin. Trisha Fox was then treated with a neural paralyzer and a truth drug called Veritazine in order for the Starfleet crew to find the hidden Scotty to treat him for the venom. With his daughter captured and being returned to normal, her father vowed to start his relationship with her anew. (TOS comic: "Who Is... Enigma?")