Troubled and uncertain, he began to fly home, back to the safety of the dragons' nest. The night fury felt as if he was teetering on the edge of something. On the edge of his own sanity, probably. But some small part of him that just wouldn't give in was wondering very seriously whether he was stumbling on the edge of something momentous...

...

"I am so beating you today!" the blue Nadder called confidently, a smile in her voice, and she passed the night fury up with a well-timed burst of speed.

"Oh, you mean the way you 'beat me' yesterday?" the night fury retorted with a laugh, matching her speed. They flew in perfect synchronization, the tips of their wings just inches from touching. A sea stack reared up between them, and they swerved smoothly around it, melting back into faultless togetherness the way the waters below them did.

"Oh, no," the Nadder grinned. "I'm winning this thing." And she folded her wings and dived, letting gravity help her speed. The night fury followed just behind her, and together they rounded the curve of a little island. She put on another burst of speed and disappeared around the curve. The night fury laughed and prepared himself to fly faster, ready to charge ahead of her. But at the last second, he caught something familiar out of the corner of his eye, and decided rather abruptly not to race at all. Because there on one of the island's high cliffs stood a lone figure in black, and the night fury knew immediately who it was.

The viking looked smaller than usual, with the island's rocky walls rising jaggedly around him. He stood completely still, hooded face turned up to the sky, watching as the night fury streaked towards him. The dragon landed soundlessly by the viking's side.

"Hiccup," the dragon greeted with a little contented warble. "Good day."

"Yes," Hiccup responded slowly, nodding. "Yes, it is." The viking sat down then, back against the salty rock wall. He pulled his knees to his chest and wrapped his arms around his legs. The dragon was struck once more by the human's smallness. He would almost seem fragile and innocent - if not for the trademark silver dagger in his hand, forever dripping a slow stream of ghastly red. The night fury wondered, for probably the millionth time, why Hiccup had this dagger. But unfortunately, this was a question the human had never deigned to answer.

"Why are you here?" the dragon asked instead, following Hiccup's gaze and staring out at the sky. It was stunningly clear today - perfect flying weather.

"I like watching you fly," Hiccup answered honestly, a fond smile in his quiet tone. "I wish I could, too."

"Humans don't fly," the night fury scoffed automatically, but even as he said it, he felt, for some inexplicable reason, that this was something wrong to say. Logic dictated that humans did not fly, could not fly, and never would. But something just beyond the edges of his understanding seemed to pull him towards the idea, until the thought of it resonated in his head like the sound of thunder in a storm. The dragon looked at Hiccup, amazement, curiosity, and confusion in his wide green eyes.

"What is it?" Hiccup asked knowingly. The night fury was sure that if he could see Hiccup's face, the viking would be smirking. His eyes would be dancing. The dragon wondered fleetingly what his human friend's eyes looked like. But the thought vanished as the viking stood abruptly.

"Are you leaving?" the night fury wondered, unable to keep the note of disappointment out of his voice. And, if he were completely honest, there was a note of anxiety in there, too. Incredibly, the strange, eerie human boy had become a kind of friend to the night fury. And each time the human vanished, the dragon was haunted by the idea that maybe he would never return. Maybe he was just an illusion, a vision, a manifestation of the dragon's own insanity. Each time the viking disappeared was just another chance that the night fury could be actually crazy. Swallowing the fear that began to make itself apparent, the dragon spoke up again, rephrasing. "You're not leaving right now, are you?" he asked. Hiccup turned and faced him, head cocking slightly as he considered the night fury before him.

"Do you trust me?" he asked.

"What?" the night fury wondered, surprised by this unexpected response.

"Do you trust me?" Hiccup repeated, moving closer.

"Well, yes," the night fury replied. "You saved my life." Hiccup was silent for several seconds.

"If you do trust me," he said finally, "you'll believe me when I say that I'll return." This, quite obviously, meant that he was in fact leaving now, and the night fury felt his heart sink. The disappointment must have been written on his face. "See?" Hiccup pointed out quietly, almost curiously. "You don't trust me." The viking seemed almost disappointed himself. "I need you to trust me." And so saying, he moved decisively forward, approaching the dragon. The space between them closed very quickly, and suddenly, the night fury found himself closer to the human than he had ever been before. The scent of wood and fire filled the dragon's nostrils. But the sharp metallic smell of metal was there, too, reminding the night fury all-too-well of the menacing dagger in Hiccup's hand. And then Hiccup moved, unseen eyes fixed resolutely on the dragon, and his other hand extended out until it remained just inches from the night fury's snout, palm open expectantly.

"Do you trust me?" Hiccup repeated. The night fury swallowed, a thousand thoughts and emotions racing through his mind at once. The logical side of his mind reminded him that this human had saved his life, and thus could be trusted. The instinct side of him wanted to get as far away from Hiccup's evil-looking dagger as possible. Another part of him begged to trust the human he had begun to consider as a friend. But the night fury could also smell the slightly metallic, sickening smell of the sinister blood that adorned the dagger. The seconds ticked by in their own individual eternities as the night fury debated, unsure what to do. Did he dare trust this human? The choice was agonizing, the tension tangible, and the gravity of the situation almost unbearable. And then...

Hiccup made the choice for him.

Without any hesitation, the viking turned his head and looked away.

The night fury was beyond amazed. Hiccup's dagger-free hand remained resolutely before him, but the viking's head was turned the other way. Not looking. Just trusting. The incredible, impossible human was essentially saying, Do you trust me? Because I trust you.

Yes, the night fury knew then, fiercely and unquestionably. Yes, I trust you. So he moved forward ever so slightly... and placed his snout in Hiccup's hand.

A feeling indescribably wonderful and warm came over him then, and his heart beat wildly. Exhilaration rushed through the dragon's veins as he relished the inexplicable bond that was so complete it felt as tangible as the rock beneath him. Eyes closed, he rested in the moment. It felt so right. Fated. Destined. Inescapable. As if this moment was always meant to happen, and always would, in every universe, in every version of their world.

Slowly, the dragon opened his eyes.

Hiccup was gone.

But he would return. The night fury knew it now, beyond a doubt.

...

Night settled upon the earth gently, like the wings of a mother dragon draping carefully around her young. The night fury smiled, pleased. He waited patiently as the dragons all curled up for the night, and wished the blue Nadder a cheerful "good night". And when she tucked her head beneath her wings, the night fury took off. In seconds, he was out in the cold night air, invisible in the comforting blackness. It was time to head back to the human village. Time for another lesson from the old viking woman. Time to understand more.

On this particular night, the woman drew a curve. And then she placed her staff back on a random point in the curve, and drew a straight line from there. It shot off away from the curve in a clean tangent line. Then, moving her staff back again to the curve, she drew a small x at the point where the tangent veered off. The woman stood back, admired her work. The night fury studied it, too, frowning in discouragement. He waited for her to draw more, but she never did. Instead, she moved forward and methodically erased it from the sandy floor. This, the night fury knew, meant that she was done for the night, and he watched in consternation as she retreated to her bed.

Flying back to the dragons' nest, the night fury thought furiously about the maddeningly vague drawing. Usually he could understand everything she drew. Each time he went, he was rewarded with more information and more knowledge. It was wonderful, exciting, addicting.

But not tonight. Tonight, he hadn't understood her "lesson" in the slightest. But the dragon was not willing to give up so easily. As the dragons' nest came into view on the horizon, the night fury decided to get a second opinion...

...

"A tangent line, eh?" the old Gronckle confirmed, nodding assuredly. "Yes, I know exactly what you're talking about."

"You do? Oh, good," the night fury rejoiced, relieved. "Could you explain it to me, please?" The Gronckle grinned at him, and a light ignited in his old brown eyes. It was the same happy look he always had when an eager listener gave him his full attention.

"I'd be glad to," he agreed. "The woman was demonstrating what is called a tangent universe."

"A tangent universe?"

"That's right. You remember the curve she drew? Well, that represents the original, normal timeline. The string of events as they were meant to happen, so to speak. Now, the little x represents a moment in time with the potential to change said timeline. When this happens, a tangent universe is created. Because of the outcome of that one moment, the universe is skewed, and the string of events is changed completely. The universe is set on an entirely different course than the one originally intended. You understand?"

"Yes, I think so," the night fury murmured, thinking it over.

"I'll give you an example," the Gronckle continued. "Let's say in the original universe - the curve, you know - I was meant to be king of all the dragons." Here, the night fury laughed good-naturedly, but didn't interrupt. "But then, we get to the x - that one specific moment. Let's say that in that moment, before I become king, I've been captured by a crazy group of humans. They say they will throw one of their double-sided human weapons up into the air. If, when it lands, the black blade strikes the earth, they will kill me. If the gold blade strikes the earth, they will let me live. Now let's say that in the original universe, it lands on the gold side. I live, and end up being king of all the dragons. However, in this moment - the x, you remember - the weapon lands on the black side instead. This creates a tangent universe in which events unfold differently as a consequence; I am killed, I do not become king, and everything happens differently. Does this make sense?"

"Yes!" the night fury exclaimed happily, comprehension dawning in a satisfying epiphany of sorts. "I understand it now. Thank you!"

"You're welcome," the Gronckle acknowledged. "Always happy to share my knowledge."

"I do have a question, though."

"Ask away, my friend." The night fury hesitated, and then voiced his concern.

"How do you know if a tangent universe has been created?"

"A very good question," the Gronckle commended. He remained silent for a while, thinking deeply, and then he spoke. "Well, as a tangent universe, the universe would be inherently different from the intended universe. And as such, I think, would be unstable. There would be... signs. Strange events. Abnormal occurrences. And disastrous crises that would lead, ultimately, to the eventual collapse of the universe."

Chills ran through the night fury like ice water in his veins as the Gronckle's words resonated within him. Strange events. Abnormal occurrences. Thoughts of Hiccup began to surface, unbidden, in his mind. He thought, too, of the odd liquid spheres. The impossible reality that time travel had become.

The eventual collapse of the universe.

According to Hiccup's countdown, time was running short. 2 days, 19 hours, 7 minutes, and 48 seconds were all that remained, all that stood between now and the end of the world.

"Are you alright?" the Gronckle asked, bringing the night fury out of his frightening thoughts.

"Yes," the night fury lied smoothly. "I was just thinking."

"I see," the Gronckle responded. "Of course," he added, "those are just my own thoughts on the manifestation of the tangent universe. I could be wrong," he explained fairly.

"Oh," was all the other dragon could manage. What was he supposed to think now? Was there a possibility that this wasn't a tangent universe, and that the end of the world really wasn't as imminent as he feared?

"But, if I may say so," the Gronckle put in, turning around and getting ready to fly back to the warmth of the nest. "In all of my years, I've never once been wrong." And then he spread his wings and flew away, leaving the night fury alone with his thoughts.

Horrendously uncertain, the night fury followed after him, and curled up on an isolated ledge of the dragons' nest. His troubled mind ceaselessly tossed around the question of whether or not they were currently in a tangent universe, and just how long there might be before the world's end. After all, he reasoned, the elderly Gronckle didn't know the signs of a tangent universe - not for sure. Maybe everything would be fine.

With a frustrated sigh, the night fury vowed to ask Hiccup about tangent universes when he next saw him. And then he closed his eyes with stubborn will and forced himself to get at least some semblance of rest. After an indeterminable amount of time, the dragon finally drifted off to sleep, and he dreamed of lights and universes and starry skies, and of moments that could have gone differently...