Charles Fischer was a member of a group of Skynet collaborators known as the Grays, whose purpose was to train Terminators how to interact with, and infiltrate humans. He is captured by Jesse Flores in the present day when she recognized him at a local mall. She then contacted Derek Reese to try to convince him of his identity.
"Future flashbacks" show that Fischer is an extremely sadistic (and possibly, the highest ranking) member of the Grays. He is shown torturing Derek Reese in front of an audience of what were Series 888 Terminators. It is obvious that he has a strong self-preservation instinct, and is willing to betray his fellow humans for the sake of his own hide; seemingly with pleasure. Like most personalities of this type, he is also a coward. 
In the present, he took on the identity of Paul Stewart, a clockmaker. He used this occupation to explain the existence of the handless clock tattoo on his arm.
Fischer was an engineer subcontractor from a company known as SRF. In another example of Predestination paradox, after hours, one night, his older self entered a secure government facility using a retinal scan lock, and accessed numerous high-security computer systems without authorization. Utilizing knowledge of future-tech programming techniques, he installed permanent back door accesses into numerous Department of Defense and national industrial complex computer systems to pave the way for the rise of Skynet, making it impossible for the other employees and authorities to access it and repair any damage that he had done. Fischer's younger self obviously had no idea that this had happened, and that his self-incriminating actions would make mincemeat of even the most airtight alibi. Because of this, the FBI agents that interrogated him had irrefutable proof that "he" had, in fact, performed this illegal activity.
Fischer is first revealed as an older man being held hostage in a shipping container at an abandoned warehouse. Jesse Flores had apparently followed him into a parking garage and waylaid him, contacting Derek Reese once she had secured him.
After being captured and smacked around a bit by Jesse, Fischer is then interrogated by Derek. He lies about his identity, even showing a driver's license as proof. He tells the partial truth in saying that he was once an engineer who got bored with his job and decided to take classes in watch repair. In reality, he was arrested for espionage and sentenced to life in prison. Derek figured this out when he noticed Fischer's tattoo; the handless clock that signified "until the end of time".
Later, Jesse also brings the younger Fischer into the shipping container, showing Derek that the two are, in fact the same man. She does so by revealing an identical birthmark shared by both men. Derek then tries to get the older Fischer to confess by pulling out a few of the younger Fischer's fingernails; an act that reveals that Derek has a sadistic side himself. The older Fischer caves, begging Derek to leave the young man alone.
After confessing to being a member of the Grays, he tells Derek that he is not in the present for any specific mission, but as a reward. Those who serve Skynet faithfully are given the opportunity to time-travel back to the past, to live out the rest of their lives in a "war-free" existence.
It is revealed that the old Fischer assists and educates the Terminators; demonstrating through drugs, starvation and other brutal acts; live mental and physical breakdowns of human subjects. Derek promps Jesse for more details of "what he had done to her". After several "future flashbacks" of both Jesse (both she and Fischer are from the same alternate future) and Derek, Jesse puts a bullet in the older Fischer's head just as Derek is about to execute the younger Fischer. The two then let the younger Fischer go, who is later arrested by authorities. We last see the younger Fischer being locked away in a cell for the crimes just committed by his future self.
- Derek: What did he do? Jess, what did he do to you?
- Jesse: There was a raid on a bunker. Metal was everywhere. They took out our command, killed everyone over 30, killed the children. They took the prisoners to this place.
- (Jesse's story in Derek's imagination)
- Fischer: (to watching T-888's) I will demonstrate...
- Jesse: He was there.
- Fischer: (to watching T-888's) How to extract information out of people. In essence, we will explore what makes people tick.
- Jesse: He said he was gonna teach them how to talk to people, How to get information from people, how to be people.
- Fischer: (to watching T-888's) There are many different tactics you can employ, all of which will give you a deeper insight into the human psyche.
- Jesse: It was a kind of perverse theater, like... Like he was teaching them.
- Derek: How long did it go on?
- Jesse: Weeks. Months. There were drugs... And starvation. And he talked to you... For hours... For days...
- Fischer: Tell me about the bunker.
- Jesse: Just to break you down, just to get to your in--
- Fischer: Tell me the location, please.
- Jesse: To get to you.
- Fischer: Tell me the location.
- Derek: How'd you get out?
- Jesse: I don't know. You never told me.
- Derek: (stunned) What?
- Jesse: (sighing) It wasn't me Fischer had, baby. It was you.
- (Jesse's Derek undergoing interrogation)
- Derek: Aah! Aah! Ah! Go to hell. Go to hell.
- Jesse: You don't remember. I can't believe you don't remember. But it was you.