"People are very strange these days." - Mark talking to Johnny about women cheating on men.Mark Reynolds Sestero was an undercover vice detective who was the best friend of Johnny. However, Mark betrayed Johnny when he had intercourse with Johnny's fiance, Lisa. This led to Johnny killing himself, which then led to Mark going on a journey around the country to analyze himself as a person, which brought him to Torgo.
Mark used to know this girl. She had a dozen guys. When one of them found out about it, he beat her up so bad she ended up in the hospital on Guerrero Street.
Lisa invites Mark to Johnny's apartment with the intention of seducing him. Mark resists, as he doesn't want to cheat with his best friend's girlfriend. It doesn't take much to make him submit however and they make love on the stairs, agreeing to keep this a secret.
Later, during Denny's encounter with the dangerous San Francisco crimelord Chris-R on the rooftop, Mark and Johnny coincidentally head up there at the exact same time. Mark and Johnny take Chris-R downstairs, but soon after, Mark tells Johnny he can handle the thug. Johnny agrees and leaves, while Mark then disposes of Chris-R's body.
Lisa calls Mark a few days later and begs him to come back to her. Mark still puts up resistance, but the next day he shows signs that he's reconsidering. Johnny talks with Mark on the rooftop about how some women like to cheat on their boyfriends, just as Johnny heard rumors that he'd been abusing Lisa. Johnny suspects that something is eating away at Mark, but Mark ends the conversation before he can reveal too much.
The next day, Johnny, Denny, Mike, and Mark are all playing football in the alley somewhere around the apartment, when Mike brings up his underwears tragedy. Mark, not hearing the story clearly, teases Mike and accidentally pushes him into a set of garbage cans. The gang takes the injured Mike home.
Johnny tells his friend Peter that Lisa has been acting distant around him just as Mark brags about seeing someone new. Peter puts two and two together and confronts Mark about sleeping with Lisa. Mark was smoking marijuana at this time and the paranoia he was feeling made him lash out at Peter before admitting that he'd slept with Lisa. He blames Lisa for all of this and Peter tells him that she doesn't really love Mark, she's just using him for thrills.
After this, the whole gang are at Johnny's place wearing tuxedos for unexplained reasons. They all head outside to play a game of football when Peter, in a sad attempt at looking cool, falls over. This fall eventually led to his death a couple of hours later. That same day, Mark and Johnny go out to lunch where Johnny asks Mark about his sex life. Mark simply shrugs.
Lisa and Mark resume their tryst, at one point even having sex in Johnny and Lisa's bed. Mark has well and truly stopped caring about Johnny's feelings by this point. The next morning, Mark and Johnny go jogging at Golden Gate Park. That same day, Lisa's friend Michelle catches Mark and Lisa in a state of undress, but laughs it off, as she believes that Johnny had been abusing Lisa and that he deserves this.
At Johnny's birthday party, Lisa tries to have sex with Mark, only to get caught by another of Johnny's friends and chastised. Mark snaps back at him. Johnny announces that Lisa's expecting and Michelle confronts Lisa over this, worried that the baby might be Mark's. Lisa assures her that she made up her pregnancy to add an extra level of excitement to everything. Mark demands to know more about this supposed pregnancy, but Lisa refuse to tell him anything. They start arguing and Johnny tries to break them up, hinting that he may suspect Mark of sleeping with Lisa. His suspicions are confirmed when he catches them dancing together. Johnny and Mark get into a fight and the party ends.
Afterwards, Lisa calls Mark and agrees to run away with him. Johnny loses his temper and trashes his apartment, before committing suicide. Mark finds Johnny's body and immediately feels remorse for driving him to this. Lisa tries to exploit this opportunity, but Mark angrily pushes her aside, confirming that they're officially over.
The Room: Mark's Vice (First chronological appearance)
The Room (First actual appearance)