Doctor Marcus O'Herlihy was a male Human scientist and medical doctor in the 21st century. He had a wife, Jocelyn, and a daughter, Tera, both of whom he was highly devoted to.

In the year 2020, O'Herlihy was a member of the USS Lewis & Clark's crew when it became the first Human vessel to explore Saturn. O'Herlihy conducted many scientific experiments during the trip but later tried to sabotage the mission because the Human Extinction League kidnapped his daughter and forced him to. O'Herlihy's first act of sabotage was to start a fire aboard ship, but the rest of the crew put it out with fire extinguishers. O'Herlihy then tried to crash the Lewis & Clark into Saturn, but was stopped by James T. Kirk who whose mind was inhabiting Shaun Christopher's body. O'Herlihy then spent the rest of the trip back to Earth tied down and upon the ship's return, he retired from NASA. (TOS novel: The Rings of Time)

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