Menos was a 22nd century Vulcan operative. He was assigned to infiltrate a cell of smugglers but eventually became one himself. He smuggled dangerous bio-toxins that were used to create transgenic weapons.
T'Pol was sent to apprehend seven rogue operatives and succeeded in capturing five of them. Menos and another rogue operative, Jossen, escaped. Several years later, she tracked them to the jungles of Risa, where she killed Jossen. Menos managed to escape again. Disturbed that she had taken a life, T'Pol undertook the Fullara, a Vulcan technique used to suppress memories.
In 2152, while serving on the Enterprise, the Vulcan High Command contacted her and informed her that Menos had surfaced again in the Pernaia system. She asked her commanding officer, Jonathan Archer, to help her apprehend him. T'Pol had unpleasant flashbacks of her attempts to arrest Menos on Risa.
After T'Pol apprehended Menos, he warned her that if she arrested him, he would be executed for a crime he didn't commit. Menos claimed that his body had been badly damaged by hauling used warp injector casings and that he never dealt in illegal cargo. After they had captured Menos, T'Pol's memory of killing Jossen returned. Menos used this as in an attempt to play on her sympathy. Menos claimed that Jossen was another innocent Vulcan who did not want to return home after a surveillance mission. A horrified T'Pol remembered shooting him. Menos asked her whether she's willing to commit the same crime against him and undergo the same suffering. With T'Pol's guard down, Menos managed to escape, and Archer, Mayweather and T'Pol chased him to his ship. He tried to escape again but T'Pol stopped him and almost killed him, but was persuaded not to by Archer, who told her that she was only to capture Menos, not judge him. A hidden stash of biotoxins was discovered on his ship and thus Menos was guilty. (ENT episode: "The Seventh").