In 1994, Frank Brennan worked for Bert Myers. It was Brennan's job to pay $3 million to a Russian general for a nuclear warhead that would otherwise have landed on the black market. But Brennan wanted the money for himself, and tried to steal it. Myers refused to go along with him, and shot at him. At the last moment, Nick Wolfe appeared, and in their struggle, Brennan was shot and died his First Death.
Over the next four years, Brennan learned the rules of Immortality, and how to fight with a sword. He, however, preferred his gun instead of his sword, and didn't fight other Immortals, when given a choice.
In 1998, Brennan appeared in Paris. He killed one of Myers' men, and also shot at Myers himself. It looked as if someone was targeting the starlet, Debra Dow, who Myers had contracted to protect. Brennan met Amanda and Nick Wolfe.
Shortly thereafter, he spoke to his former partner and lover, Rachael, who had thought him dead. He told her that Myers and Wolfe had lured him into a trap. Later on, he met Amanda on Holy Ground. She tried to threaten him, but he didn't care. When Myers entered the church, Amanda beat Brennan down, and hid her meeting from Myers.
Rachael then took Brennan to Nick Wolfe, which led to a chase over the rooftops of Paris. When Brennan jumped off the roof, he got up and ran away. Rachael saw this, and didn't want to help him anymore. Believing if she weren't for him she was against him, he killed her.
The next day, he lured Amanda into an old factory building. They fought, and Brennan was defeated. He pulled a gun and shot Amanda, but before he could beheaded her, he was shot in turn by Nick Wolfe, after which, Amanda took his head.
- Brennan was Immortal for only four years.
- The actor who portrayed Brennan, Michael Siberry, also appeared in Highlander: The Series, where he portrayed Martin Hyde.