Disclaimer: I do not own Naruto or any of its characters.

Warning: This story contains spoilers for the manga.

The First Heart

Side by side, father and son fought. Although their opponent was a lone ninja, he was one of the most powerful men alive, and was giving them quite a bit of trouble.

The ground split open, and seedlings burst from it, growing longer and thicker until they had become mighty trees. Branches and roots whipped out, seeking to entangle the two shinobi who stood back-to-back.

"Are you all right, Father?"

"I think so. That last branch gave me a bit of a gash on my arm, but it's nothing serious." The older of the two men held a katana in front of him in a defensive position, using it to cut through any branches that came their way. "Are you all right, Kakuzu-kun?"

The younger man nodded, green eyes flashing angrily. "Don't worry, Father, we'll beat him."

Their opponent stood before them, surrounded by the gigantic plants he had created. He was the man who would one day be known as the Shodaime Hokage. He and his brother had created a new ninja village in the Country of Fire, gathering all of the most prominent shinobi clans in that nation into one place. Needless to say, many of the nomadic shinobi clans in the surrounding lands opposed this action—they didn't like the idea of so many potential enemies being allied together. Kakuzu's clan came from a territory north of the Fire Country, a place filled with flowing rivers and majestic waterfalls. The elders of the clan had chosen to openly oppose the Shodaime's new alliance of ninja, and had sent several of the clan's best warriors to defeat him.

Unfortunately, the Shodaime had proved a more difficult opponent than they expected, and Kakuzu and his father were the only two of that force still standing. "Shodaime!" Kakuzu's father called. "The other shinobi clans will not allow your village to survive. To gather so many powerful clans together in one place upsets the balance of power. It cannot be allowed!"

"That balance of power is the reason why the wars between the clans never end!" the man called Shodaime shot back. "How many people, both shinobi and civilian, have died in those eternal conflicts? Bringing the clans together is the only way to end the bloodshed! Why fight us when you could join us?"

"Join you? And lose our independence, become subservient to your will?" Kakuzu sneered with contempt. "The Kuriyama clan will never submit to the rule of others!"

"We do not ask you to 'submit' to anyone, merely to leave behind the barbarism and cruelty of the past. We shinobi have been warlords, imposing our will on others by fear. Instead, we now have a chance to become protectors and guardians. Is that not a better way to live?"

"Enough of this!" snapped Kakuzu's father. "I, Shiraishi, the heir of the Kuriyama clan, together with my son, will put an end to this idiotic 'dream' of yours once and for all!" Putting force behind his words, he formed a series of hand seals. When the next branch rocketed towards him, he didn't block it with his katana. Instead, he grabbed it—and the branch shriveled. Like most members of his clan, Shiraishi was a master of water element ninjutsu, and he was draining every molecule of water from the massive trees created by the Shodaime's legendary jutsu. All around them, towering redwoods collapsed to the ground, spindly and dead.

Kakuzu grinned, and formed seals of his own. As he sprinted towards the Shodaime, water flowed and swirled around his right hand, forming itself into a blade. Kakuzu swiped at the Shodaime, who dodged backwards. Kakuzu growled, and the blade of water elongated, pursuing the Shodaime. The other man continued to dodge, elegantly avoiding each blow. But Kakuzu's weapon continued to change its size and shape, relentlessly attacking the Shodaime no matter how fast he moved.

Now Shiraishi joined the attack as well. With one hand, he slashed at the Shodaime with his katana, and he held a shield of water in the other. The Shodaime's expression turned grim, and he raised his hands in front of him. A thin tendril of wood sprang from each fingertip. The tendrils joined, forming a vaguely humanoid shape. The wooden automaton's features became more defined, until the two Kuriyamas were facing a flawless copy of the Shodaime. As the elaborate dance of attack and defense continued, it became impossible to tell which was the real Shodaime and which was the clone.

Shiraishi continued to fight one of the Shodaimes, blocking every attack with his water shield. As the battle continued, he moved more and more slowly, as though the shield on his left arm were becoming heavy. Finally, he raised the shield too slowly, and was knocked to the ground.

"Father!" Kakuzu called out, but then saw that his father was smiling. It's a ruse, he realized. Sure enough, his father twisted and grabbed the Shodaime by the ankle. One touch. That's all he needs. Just as had happened to the trees, the Shodaime withered and fell to the ground as every molecule of water was ripped from his body. As he fell, his body lost its definition, and only a formless lump of crushed wood hit the forest floor. That one was the clone! That means that the one I'm fighting is the real one!

Distracted by his father's fight, Kakuzu wasn't able to react in time to prevent the real Shodaime from launching a new attack. Wooden tendrils once again extended themselves from his body—but not just from the fingernails this time. Instead of forming a clone, the tendrils surrounded Kakuzu, binding him in place. Then, to Kakuzu's horror, the tendrils began to grow thorns, cutting cruelly into his body. He cried out in pain, and struggled to break free.

"DON'T YOU HURT MY SON!" Shiraishi roared, and leapt at the Shodaime with renewed vigor. Jets of water blasted up through the ground, tearing through the tendrils and knocking Kakuzu and his opponent apart. Kakuzu fell to the ground. His vision faded in and out of focus, and every limb burned with agony. Then the pain began to slip away, replaced by a cold numbness. That is not a good sign. Looking down, Kakuzu saw that a ragged hole had been ripped in the upper left part of his chest. My heart…

"Hey! What do you think you're doing, bastard?!" Kakuzu's spirits lifted at the new voice. Leaping over the dead trees, several figures approached, all displaying the ebony hair and vibrant green eyes that Kakuzu and his father shared. The Shodaime looked up, eyes narrowing at the new arrivals.

The woman who had spoken rushed to Shiraishi's side. "You okay, Brother?"

His father nodded tersely. "I'm fine, Shimada-san, but Kakuzu-kun is badly injured."

Once again, wooden tendrils emerged from the Shodaime's fingers, whipping through the air towards Kakuzu's aunt. She jumped into the air, flipping and twisting to avoid them. Nor was her movement random—it brought her closer and closer to the Shodaime, until she was able to aim a kick at his head.

The Shodaime dodged Shimada's kick, then glanced from one shinobi to another. He seemed to realize that he was distinctly outnumbered. Still, he wasn't the type to just run away.

With the Shodaime distracted by his siblings, Shiraishi took the time to examine his son's injuries. "Yamamoto-san, I can't treat these wounds in the middle of a battle. We need to get away from here."

The oldest of the clansmen who had come to the father and son's aid nodded. "Understood." Yamamoto then performed a technique that would later be copied by the ninja of the Mist Village: the Hidden Mist Technique. Thick fog rolled in, forming an obscuring veil between the Shodaime and his enemies. He tensed, expecting an attack, but none came. By the time the mist cleared, the Kuriyama clansmen were gone.

"Don't be afraid. You're safe now." Kakuzu could hear his father's reassuring voice, but his vision was too blurry to see the man's face. "There's a lot of internal damage. I'm going to fix it as best I can." Kakuzu had the utmost confidence in his father's abilities—the man was well-known as an outstanding medical specialist, after all.

Shiraishi pulled a roll of thick black thread out of his supply pack, laid it on the ground, and rested his hands against it. He infused the surgical thread with his own chakra, and guided it into his son's wounds. The chakra-filled thread moved through Kakuzu's body on its own without the need for a needle or any surgical instruments. It stitched blood vessels, patched up ruptured organs, and knitted broken bones back together. Spleen, ribs, kidneys, right collarbone, left lung—all were fixed without too much trouble.

Then Shiraishi saw how much damage had been done to his son's heart. His shoulders stiffened, and his previously hopeful expression shifted to one of dismay.

"What's wrong?" asked Shimada.

"The injury to Kakuzu-kun's heart is extensive. It's a miracle he's still alive."

"But you can fix it, can't you? You seem to have healed everything else."

"The heart was damaged more badly than the other organs. I can't…I can't do anything to fix it. It's simply too injured to function. The only possible way to save him would be to completely replace the heart."

"But…that would require another heart to replace the damaged one with."

Yamamoto had been kneeling at Kakuzu's side; now he stood up swiftly. "I'll go back to the scene of the battle and retrieve a heart from one of the corpses."

"No, that won't work," said Shiraishi, and everyone could hear the desperation in his voice. "Once it's been deprived of oxygen, the tissue rapidly dies and becomes useless. If we were going to replace his heart, the donor heart would have to be one that was still living."

"But that means it would have to be taken from a living person," said Shimada in a tone of shock.

"Correct."

The clansmen all looked at each other, and it seemed to Shiraishi that all of them opened their mouths to speak at the same moment. In those days, before the founding of the shinobi villages, each clan was a social and political entity unto itself. Loyalty to one's clan was everything, and the bonds that tied clansmen together were exceedingly hard to break. It was, therefore, not so surprising that all of the men and women gathered there were willing to give their lives for the young man who lay dying before them.

Shiraishi knew this, but he also knew that it was a father's responsibility to protect his child, no matter what the cost. So before any of his relatives could actually say what they were planning to, he declared, "I'll do it."

The others stepped forward, protesting, but Shiraishi didn't listen. He flowed more chakra through the threads in Kakuzu's body. Those threads rose from Kakuzu's chest and punctured Shiraishi's, burrowing through flesh and bone until they reached the beating heart. The threads wrapped around that organ and ripped it from its owner's chest, pulling it down to Kakuzu. The pain was agonizing, and even a hardened shinobi like Shiraishi couldn't help but cry out. Just a few more minutes. I must stay alive long enough to put the heart in place. Slowly, the threads lowered the heart into Kakuzu's chest, attaching it to the blood vessels in the proper places. Blood began to pump through Kakuzu's body again, and his erratic breathing steadied. Shiraishi laid his hands against his son's ruined chest, healing the last of the damage.

Kakuzu groaned as his vision started to return. For a while there, he had thought he was going to die. All sensation had left his body, and the voices of his kinsmen had seemed to be receding further from him every moment. Now, he heard shocked voices whispering and murmuring all around him. His body throbbed with pain, and blood loss made him dizzy when he tried to sit up. "Take it easy, Kakuzu-kun, you've been badly hurt," said a kindly voice. He recognized Shimada as she helped him into an upright position.

Kakuzu felt strange…as if something was moving inside him. Raising a hand to his face, he shouted in fear. A long cut down his left arm had been stitched together…and the stitches were twitching. He could feel the chakra flowing through them just as it flowed through his biological tissues. "What's happened to me?"

Shimada's brow furrowed, and she seemed reluctant to speak. Kakuzu saw that her eyes were red, as though she'd been crying. "Your father saved you, Kakuzu-kun. Those are his threads. They've repaired all your injuries."

"But why are they moving?"

"Kakuzu-kun…there was severe damage to your heart. Too severe for even your father to heal. The only solution was a transplant. Your father…he gave you his heart. Since the heart is the center of the chakra circulatory system, I think that the chakra stored in his heart must be giving you the ability to animate the threads in the same way that he was able to."

"But if he gave me his heart…that means…" Kakuzu turned then, and saw the body of his father lying beside him. There was a gaping wound in the older man's chest, and his eyes stared blankly up at the sky.

Kakuzu was a ninja, and like most ninja, he had developed a fairly thick skin. It took a lot to upset him. But he felt hot tears running down his face as he gazed at his father's body. Bowing his head in grief, he saw something unusual on his chest. On the upper left side, right over the position of the heart, was a mask. It was made of bone, as if it had grown out of his ribs. It seemed to be protecting the heart that Shiraishi had transferred into him, and something about its features reminded Kakuzu of his father's face. Did he craft that with his last breath, as if he wanted some part of himself to continue protecting me, even after he was dead?

Just over a century later, as the Copy Ninja's Chidori tore through the the last of his five hearts, Kakuzu thought of his father. In the decades since that fateful day, he had changed greatly. His clan had lost their fight against the Shodaime and his followers, and most of the people he loved—Yamamoto, Shimada, and many others—had died. He had eventually discovered that he could steal hearts from his enemies to keep himself alive, and he had seen all of his precious people die while he continued to live. Caring deeply about people only to lose them, over and over again, became so exhausting that he finally stopped caring about anyone at all. At that time, he had abandoned his home and become a missing-nin. Yet now, he remembered his family and the bonds they had shared. As his vision faded, he knew that unlike the first time he'd faced death, there would be no miraculous rescue this time. His last coherent thought was of the man who'd made the intervening decades of life possible.

Father…
A/N: Like every other Akatsuki fan, I was kind of annoyed that Kishi killed off Kakuzu without explaining anything about his background—like, for example, how he ended up with that weird tendril/thread stuff inside him and how he got the ability to steal hearts. So I decided to write my own background for him, and this fic is the result.