"Open your eyes. They revel in violence. They devote what little technology they have to devising ways of killing each other."
"So did we centuries ago. They just haven't realized their potential yet."
Mestral was a crewmember of a small Vulcan survey ship that crash-landed in Carbon Creek, Pennsylvania, a mining town, in 1957. Through a game of pool and confidence he had about his knowledge of simple geometry, he was pivotal in obtaining currency and, consequently, "cryogenic" (frozen) food for the crew. One job he held while on Earth was that of a coal miner.
Of the three Vulcans, Mestral developed a much stronger fascination for Human culture than T'Mir or Stron. He became romantically involved with Maggie, the owner of a local tavern. On the pretext of leaving the Vulcans' apartment in order to retrieve a phase discriminator, he went to a baseball game with Maggie to watch her son play. It was demonstrated there that he began to pick up Human traits, such as lying, whereas Vulcans ordinarily do not lie. He did not have a pessimistic view of Humans like his crewmates did. He saw them as empathetic and compassionate, and having great potential. When miners were trapped during a mining accident, he persuaded T'Mir to let him use a phaser-like particle weapon to free the trapped miners. He became a local hero after this incident. He was so fascinated with Earth that he chose to remain there, rather than return to Vulcan with his crewmates when the rescue ship D'Vahl came. Officially, T'Mir and Stron reported that Mestral had perished in the crash of their ship. (ENT: "Carbon Creek")
Mestral's history on Earth is revealed in the novels From History's Shadow and Elusive Salvation, which sees him temporarily time-travel forward to the 23rd century in the company of Roberta Lincoln, assisting James T. Kirk and Spock in investigating alien activity on 20th century Earth, although he returns to the past to continue his studies of humanity.
Mestral's name is a reference to George de Mestral, the inventor of Velcro, a product which T'Mir sold at the end of "Carbon Creek". In the episode's final draft script, Mestral was described as younger than Stron.