Disclaimer: I do not own Soul Eater, nor do I profit from this. I am simply writing this for fun.

A/N: Here is my second fic of Stein x Marie. As stated in the description, it will directly follow events in the manga, so there will be spoilers for those who haven't read it. The first chapter, and a portion of the second, are pretty much my own take on 'Chapter 40: Decision'. After that I'll explore the time Stein and Marie spent hunting down Joe's killer and so forth. Each chapter will involve a quote of some sort at the beginning that I believe speaks to the characters or is relevant for that particular scene I'm writing (kind of like Criminal Minds lol). Likewise, each chapter will alternate from Stein's and Marie's POV (ie. No double-POV chapters). I'm enjoying writing this, so I hope you enjoy reading it. So without any more of my exposition, I present the first chapter of Chaos Theory.

"Chaos, with reference to chaos theory, refers to an apparent lack of order in a system that nevertheless obeys particular laws or rules; this understanding of chaos is synonymous with dynamical instability. The two main components of chaos theory are the ideas that systems - no matter how complex they may be - rely upon an underlying order, and that very simple or small systems and events can cause very complex behaviors or events. This latter idea is known as sensitive dependence on initial conditions."

- (Definition of Chaos Theory, from Computing Fundamentals)

Part I: Departure

There was a symphony in his head; his thoughts were the orchestra, his madness was the conductor. The string instruments screeched out of tune in the left ear, while the brass instruments blared harshly in the right. However, it was percussion that dominated the fray, the drums booming in the back of his mind while cymbals crashed together in the front. Stein obediently followed the shadows of his captors, as he was consumed by the musical discord in his mind. He laughed because they could not hear it, they could never hear it. They could never understand the poignant masterpiece inside his head, or the way it almost brought him to tears. He was the single figure in the sea of theater chairs, the only person in the audience.

The symphony began to collapse on itself, the sounds distorted until all was quiet, aside from the acute ringing in his ears. It was then that he realized they were in the graveyard, rather than the death room—where he was to receive punishment for a crime he did not commit. What sort of game were they playing with him? He glanced from Sid to Spirit, every fiber of his being fully alert and ready to react in defense, as he asked of their intentions.

Then she appeared from across the graveyard and he knew, he knew it was true what they'd said. Her eyelid was red and swollen, she had been crying. Joe Buttataki was dead, murdered. His expression fell as he stared at her, feeling the intensity of the heartache in her soul. Her pain had a sort of sobering effect on him, the madness in him calming as he focused all of his attention on her. Marie, who possessed a naturally cheerful disposition, was being devoured by melancholy. He didn't know how or why, but that look on her face made him grieve for the loss of her happiness. His own pensive demeanor displayed less than a tenth of his misery. His voice was somber when he finally said her name, "Marie," though she would not look up from the ground.

He felt a furious, violent rage build up in his body—but he also acknowledged the futility in it. There was an upheaval of ire trapped within his skin, and simply put, nowhere to expel it. He remained still, though his fists trembled, as Spirit relayed the details of the crime to him. He wanted to punish the culprit for this gross offense against Marie. He wanted to murder the person who had put that look on her face. However, a simple death would not suffice, no. He wanted to tear that person limb from limb, give them a slow death that would be agonizing until their last breath escaped their shattered teeth and broken lips. Although his imagination horrified even him, it was necessary, evil but necessary. He was responsible. He had failed Marie, his partner and friend, in his relapse into madness.

Stein's gaze briefly met Spirit's, as the death scythe informed him that he was to covertly investigate Joe's murder, clear his name and bring about justice. It was difficult for Stein to believe that after everything he had put Spirit through, the weapon remained a dependable comrade and friend. He offered a slight nod of acknowledgement when Spirit had finished explaining the plan, his eyes sliding back to peer at Marie as Nygus spoke with her. "What will you tell Lord Death? You're all putting yourselves at risk, doing this thing for me." He asked Spirit, without looking back at him.

Sid conveniently found something to look at in the distance, as he pretended not to eavesdrop on the conversation between Spirit and him. Stein could feel Spirit's eyes scrutinizing him as he answer his question, "We're going to tell him that you escaped capture, which is easy to believe, and that you abducted Marie in the process."

"Why would I take Marie with me?" Stein responded, still staring at her. He could hear what Nygus was saying about him and he thought that the parallels in the conversations were ironic. Here Spirit was, trying to convince him to go, and there was Nygus, trying to convince Marie to stay.

"He'll believe it easily, Stein."

"That doesn't answer why, Spirit."

"Does it matter why?" Of course it mattered why. Stein always wanted to know they answer to three main questions: what, how and why. The other man sighed heavily, although Stein did not know what it was about this conversation that was taxing for him. Spirit allowed a silence to stretch out, stalling so that he wouldn't have to answer. But Stein was ruthless in his curiosity, and Spirit's resolve waned. His friend grunted in frustration, before muttering sharply, "He'll believe me because he knows as well as anyone that people take extreme measures when they love someone."

The pits of his grey-green eyes shifted back to Spirit, staring without blinking. There was no humor in his voice, and he remained unsmiling, as he said, "Don't make me laugh, Spirit. You and I both agreed I'm incapable of understanding such things. Stein doesn't love." He repeated what Nygus had said, though there was a hint of self-contempt in his voice as he did so.

"It wouldn't be the first time I was wrong about something." Spirit shrugged his shoulders dismissively—attempting to make light of the conversation. He added in a more sympathetic tone, "It's obvious in the way you're looking at her now. I've never seen you so upset."

"I don't know what you're talking about, Spirit." He lied, but quieted as Marie's footsteps approached him from behind. He no longer felt the prickle of madness in her presence, but instead that familiar serenity she brought about him. He closed his eyes and savored her nearness, having been unable to bear close proximity with her just days ago. The cool tranquility started in the back of his head and trickled down his spine, easing the tension from each and every nerve as it did so. It was damn near euphoric, and if he'd had a choice, he would have gladly drowned in her wavelength. The symphony in his mind was tempered by her presence, and the ringing was replaced by a soft tune similar to that of a harp.

Was that love he felt for her? This muddled emotion that throbbed in the center of his chest and made his body feel as though it might tear at the seams. There was a great, inexplicable joy that embraced his soul whenever he was with her, but he also suffered for it. Although he was content that her wavelength was back to normal, it distressed him to see her soul being suffocated by her own grief. He wanted to protect her from unhappiness, but he was at a loss of how to do so. This unfathomable regret equaled his joy, and left him with nothing but a bittersweet numb. His inner tumult caused his bones quake with a terrible anger again—but the delicate pressure of her hand against the center of his back seemed to rescind all of that and he felt a sense of peace once again.

Her voice sounded tired and heavy as it reached his ears, "…and I promised Joe I would watch over this person, let's go Stein."

Why? Why was he disheartened by the fact that she only followed him because of a promise she made to her dead lover? Why did he feel the unfamiliar sting of jealous resentment? He felt a frown tug at the corners of his lips, but he tried to remind himself that it wasn't right to be possessive of Marie; no matter what positive effect she had on his madness. She had loved Joe, not him. Marie could never love a man who was so fundamentally different from her ideal mate. She would fulfill the basic necessities of her promise, and although they would likely remain friends, she would meet another man and move on with her life. He told himself that it was better this way, in spite of how sick he felt with envy.

The words Nygus had spoken to Marie echoed in his mind, and he couldn't help but agree with her. From the instant he had drawn his first breath, he hadn't understood how to love or receive it in return. No amount of yearning or devotion for Marie would change that. It was in her nature to accept him for what he was, but she could never bring herself to love him the way she had loved Joe, he knew that much. It was not love he felt for Marie, just a combined sense of admiration and remorse.

He would avenge the murder of the man she had loved. That was to be his penance for his failure and dysfunction. He would inflict a suffering so tremendous, so absolute. He would wield Marie as a weapon of justice and he would smite them into oblivion. He felt self-condemnation for delighting in his violent sense of justice, but he knew it was what had to be done. No matter what secrets he might uncover, no matter what consequences he might face, and no matter what damage might be done to himself—he would exact retribution for her sake, and he would show no mercy. It was the unspoken promise he made to Marie, the only person in the world capable of making him feel both monstrous and compassionate simultaneously.

But for the time being, he focused on putting one foot in front of the other, as Marie fell into step beside him. They walked in absolute silence without looking at one another. The wind blew and the branches whispered with the eerie words of the dead, Marie's shoulders shook with a shiver. There was a grim suspicion in the pit of his stomach that this journey would not end well, and he thought Marie must feel it too. Even so, the pair followed their path without hesitation.

A/N: I was going to write a later chapter as a one-shot, but upon re-reading Chapter 40 of Soul Eater, I was inspired just by Stein's expression to write a full story (honestly, the look on his face nearly breaks my heart and anyone who says he's incapable of caring for Marie needs to study that chapter harder). I also originally thought the title Chaos Theory was just a nifty idea, because Stein is a scientist, but upon researching it further I found that it kind of applies to him in a way. Given that while he is chaotic, he nevertheless follows a sense of law and order by siding with Lord Death. Anyways, this story is going to have little to no fluff. It will be pretty dark and dreary for the most part. What can I say, I like my angst.