"God, why did you do this to me?"
-Mark after sleeping with Lisa
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.
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 about how some women like to cheat on their boyfriends, just as Johnny heard rumours 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.
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.
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. Lisa's friend Michelle catches them 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.
Mark's Beard Edit
Mark shaves his beard late into the film. This is given considerable attention by both the characters and the film, though no real explanation is given and it has no noticeable significance to the plot. One thing that is noticeable is that clean-shaven Mark isn't as decent as bearded Mark, embracing any chance to sleep with Lisa as opposed to keeping her at arm's length.
According to Greg Sestero, the only reason Mark shaves his beard is so that actor and director Tommy Wiseau could call him "baby face".