Mark
Mark
[Cueball and Science Girl are talking.] Science Girl: What's that on your arm? Cueball: The mark of a secret society.
Science Girl: If it's secret, why tell me- Cueball: Because I know nothing. I can't betray them because I don't know who they are. I was chosen by an agent 20 years ago. That was my first and last direct contact. Cueball: It's safer that way.
Six years later I found a piece of paper in the street with an address on it. The next day I found a can of kerosene in my garage that I'm sure I never bought. [The panel represents these actions by highlighting the mentioned objects in a world of gray.]
I didn't know whose house it was. I just knew that I'd been given my orders. And I carried them out. [A dark figure holding the kerosene is silhouetted against a flame.]
Cueball: I don't know who or what we're fighting. Cueball: Maybe we're the bad guys. Cueball: It doesn't matter to me.
Cueball: It's enough to know that there are forces working beneath the chaos of life, and I'm a part of them.
Cueball: That whatever this "Pen Fifteen" club is, Cueball: I'm in it.

In this comic, Science Girl asks Cueball about a mark on his arm. He apparently believes he is part of a secret society, so secret that he doesn't know anything about the society. His belief in the existence of the society, and that he is a part of it, stem from one contact with an 'agent'. Most people would immediately dismiss the idea of such a secret society, especially with no evidence of its existence, and no knowledge of the goals or even whether it is inherently good or evil.
Six years after being 'chosen', Cueball finds the scrap of paper with an address on it, and the can of kerosene . Both of these events are not unlikely, and easily explained as simple coincidences, but Cueball somehow sees this as a command that he must burn down the house. Cueball shows that he is willing to put other people's lives at risk, destroy property and possessions, and face the possibility of prison, all because of one event six years prior.
Cueball's belief in the society, his delusional linking of the address and kerosene, and his actions in burning down the house, show how badly he wants to be part of something bigger, and to find meaning in the "Chaos of Life".
The punchline refers to an old grade school/middle school prank (Urban Dictionary: pen 15 club , Pen 15 .) You'd typically walk up to an unsuspecting schoolmate and ask them if he wants to join the Pen Fifteen Club. You'd tell them that to join, you merely have to write the club name on them. You'd then write "PEN15" on their hand or arm, and everyone would laugh at them because it looks like "PENIS". (In a common variant, it is simply called the Pen Club, 15 is the victim's "member number", and the pranksters write "PEN13" and "PEN14" on themselves.) In this case, Cueball fell victim to this prank as a child without ever figuring out the joke, and the ink somehow never got washed off by showers or baths or removed by shedding skin. In reality, it would be unlikely for such a mark to last for so long. While methods of making someone's skin more permanently do exist, it is hard to imagine someone tattooing or branding "PEN15" on their friend's arm as a prank.
As for the title text, solipsism is the philosophical idea that only your own mind is sure to exist while other minds can't be really known and so those other minds are not proved to be real. In this context it might mean that the only one who can conspire would be you, hiding the truth from yourself.
[Cueball and Science Girl are talking.] Science Girl: What's that on your arm? Cueball: The mark of a secret society.
Science Girl: If it's secret, why tell me- Cueball: Because I know nothing. I can't betray them because I don't know who they are. I was chosen by an agent 20 years ago. That was my first and last direct contact. Cueball: It's safer that way.
Six years later I found a piece of paper in the street with an address on it. The next day I found a can of kerosene in my garage that I'm sure I never bought. [The panel represents these actions by highlighting the mentioned objects in a world of gray.]
I didn't know whose house it was. I just knew that I'd been given my orders. And I carried them out. [A dark figure holding the kerosene is silhouetted against a flame.]
Cueball: I don't know who or what we're fighting. Cueball: Maybe we're the bad guys. Cueball: It doesn't matter to me.
Cueball: It's enough to know that there are forces working beneath the chaos of life, and I'm a part of them.
Cueball: That whatever this "Pen Fifteen" club is, Cueball: I'm in it.
