During Enterprise's mission in the Delphic Expanse, Masaro was among many crewmembers who were wounded in the Battle of Azati Prime in early 2154, but was discharged from sickbay by Doctor Phlox shortly thereafter. Later the same day, Phlox, in an attempt to cheer Captain Archer up, mentioned to the captain that he had discharged Ensign Masaro that morning. (ENT: "Damage")
Unknown to anyone at the time, Masaro was also a member of the xenophobic extremist group Terra Prime. In this capacity, Masaro supplied to John Frederick Paxton, Terra Prime's leader, genetic material of Commanders Tucker and T'Pol in 2155, for the purpose of creating a binary clone, with which to further their agenda of Human superiority.
When Captain Archer decided to lead a small team to infiltrate Paxton's base on Mars, Masaro covertly sabotaged the team's shuttlepod, nearly causing them to crash. Later, when it was discovered that Masaro was the saboteur, he confronted Captain Archer with a phase-pistol. Apologizing to Archer for his actions, Masaro put the phase-pistol to his own head and committed suicide. (ENT: "Terra Prime")