- "Walter, do you know why the wolf spider is a Darwinian marvel? Because it lives alone. That is what makes it superior. It depends on nobody. Remind you of anyone?"
Mark Collins is a radio expert and former member of Scorpion. He "went down the rabbit hole," meaning that he fell away from their everyday life, and got completely lost in his mind. Walter had him incarcerated in a mental asylum, but after Mark abducted Toby, he was sent to prison.
When Mark was a child, he would stay with his uncle in South Carolina, and it was the only place he felt safe because his uncle was the only person who fully accepted Mark for who he was.
The team reunites with him when he discovers a nuclear threat over his radios, and they have to stop it. He causes discord among them, as they are upset Walter took the case without telling them in advance. After the reactor is successfully updated and shut down, Collins is put in a wheelchair for his leg wound and has one last meeting with Walter, at the end of which he remarks "I'm going away again, but I'll get out. Maybe I'll see you back down the rabbit hole". Walter then leaves and Mark is sent back to the asylum.
Mark is at a competency hearing, but Toby testifies against him and provokes him, causing a recess. He returns at the end, however, kidnapping Toby in a van.
When Toby wakes up, Mark remarks he was out for a long time, then proceeds to tell him he was working on BBI, or Brain to Brain Interphase, when he saw an obituary for Megan O'Brien-Dodd and realized Walter must have done some BBI research to try to save her, and wants Walter's research to complete his own. After relaying his demands to the team with a deadline, he sets up jumper cables to the metal chair Toby is tied to as if to electrocute him. However, when he calls Scorpion again for missing the deadline, he uses manipulated audio from Toby shouting in excitement on The Price is Right to make it sound as if Toby is screaming in pain. Walter figures this out, and the next phone call he offers to meet face to face with Mark, who declines. Mark then makes acid, and after taunting from Toby, looks through his satchel and sees the ring for Happy, and says Happy cannot marry him. Toby has knocked over the bottle of ammonia however, so Mark opens a window and pours the acid into a mason jar. He grants Toby's request to call Happy, on the condition he must tell them Mark beat them. Mark then sets the trap: He makes a web of dental floss around Toby, and the mason jar of acid is hanging over his head from a slightly sawed string, Toby has very little time left. Team Scorpion arrives, but Walter is the one to track down Collins while the others gather around the web of floss, and hits him with a pipe.
Mark taunts Walter, saying the law of the greater good dictates Walter keep Collins detained, because "I can hurt more people than Tobias could ever help". Eventually, Walter rushes to save Toby, and Mark escapes. However, Walter, Cabe, and Tim arrest Mark when he's trying to flee via bus, and Walter says because Mark mentioned a wolf spider, the state arachnid of South Carolina, that reminded Walter of the time Mark told him about the week he spent with his uncle in South Carolina, his uncle being the only person who accepted him and made him feel safe. So when Mark was going to be on the run and afraid, Walter figured he would go to South Carolina. Before Mark is taken away, he laments that "They've ruined you".
Toby visited him in jail, and in honor of the Riddler, Mark gave him a riddle about the identity of Happy's husband. The riddle includes "The water doesn't lie still, but the water still lies. Add 50 and your answer will be revealed".
Mark is very troubled person, and a master mental manipulator, able to make even Scorpion doubt both each other and themselves. In addition to this, he's insane, unstable, and has few morals, as seen when he was willing to kidnap and kill Toby, yet he didn't torture him, but instead made it sound like he was. He's extraordinarily emotional, as he waited for years to get revenge on Walter for locking him away in an insane asylum, he was even willing to let a nuclear reactor meltdown to get his revenge.
However, after being locked up for the second time, Mark seems to have calmed down a little, only targeting Scorpion as opposed to endangering unrelated parties, like all the people who would have been killed by the nuclear blast that he was previously okay with killing. Mark has little but contempt for Walter because Walter is trying to become more human, and is obssessed with proving he is smarter than him, yet curiously expressed sincere regret and disappointment when Walter ultimately decided to let him go and save Toby, choosing humanity over logic.
- He believes nearly every conspiracy theory.
- He has a 190 IQ.
- He admires the Batman villain the Riddler.