Ash Ketchum loved his Pikachu. But as he got older, he realised he really loved his Pikachu. He couldn't bear the thought of ever being separated.
But Team Rocket knew this, and they weren't going to let this observation slip them by. So they devised a plan. They would send in someone under the guise of being a scientist and offer Ash an opportunity: they would find a way to make it so that Ash and his precious Pikachu would be one forever. They wouldn't tell him that once their plan was complete, they would kidnap the Pikachu and make it theirs, successfully completing their goal.
So when Ash was approached by the surreptitious scientist, he leapt at the chance. It wasn't until Ash and his companion were strapped down to the cold metal of the operating table that he began to grow weary. "Uh, doc?" he said in his raspy voice. "Are you sure this is going to be okay?"
"Relax, Mr. Ketchum. You're in good hands." With that, the scientist attached a gas mask to Ash's face, and Ash felt himself fade away into unconsciousness.

He woke up some time later with a start. Looking around, he noticed his Pikachu was nowhere to be found. "H-hey! Where's my Pokemon?" he called out.
"Silly boy," Ash heard the voice of the scientist speaking. "You are your Pokemon now. Look and see."
Ash pulled himself up from the operating table and looked at his reflection in the mirror. He saw his face staring back at him, but to his horror, he noticed the ears and tail of his beloved Pikachu were attached to his body. A quick wag of the tail proved that hey were now really his.
"What did you do to me!"
"We made you better, dear boy. After all, you did say you loved your Pikachu. what better way to prove that than to share a body? Now be silent and watch as your changes complete."

Hours later, two Team Rocket grunts were charged with hauling out a very feisty Pikachu trapped in a cage. "Be careful!" Giovanni called out from where he watched. "It's still fresh in its new form. Make sure he's sent straight to Oak, he'll love to see what we've accomplished."