Conan sat dumbfounded on the chair for a minute or two after Agasa left. Everything happened so fast that he didn't even have the time to comprehend what's going on. He looked at the small box and sighed. I guess this box holds the answer to everything.
As Conan undo the ribbons, he laughed in disbelief. He couldn't believe the Professor actually wrapped this thing all nice and pretty. How misleading, he thought. He then took out the cap and couldn't believe what he saw inside. If he was dumbfounded earlier, he was now astonished to the point that if Sherlock Holmes walked into his house he would probably react like it's the most normal thing in the world. The moment he saw it, he knew what it was. He couldn't believe Haibara and the Professor was working on this without even dropping a hint to him. Conan was so overjoyed and excited that the Professor's last words before he left slipped his mind.
He swallowed the pill after he got over the fact that an antidote is sitting at the palm of his hand. He's so excited to get his original body back that for such a clever detective he did not notice the note slip out of the case when he opened it. He just simply walked to his kitchen all giddy to grab a cup of water. He gulped the water down as he thought of how to surprise Ran. He laughed out loud at the image of Ran looking so dumbfounded when she sees him. He walks upstairs whistling to get a change of clothes when he finally gets his body back.
Aha, I haven't seen these clothes in a while. Conan continues to rummage in his closet for clothes. When he found exactly what he's looking for, he glanced at the clock and realized four hours had already passed. Four hours?! Four hours and I'm still—
His thought-process got interrupted by the sudden tightening in his chest. He laughs despite the agony he feels. He knows exactly what's going on. He thought about exactly how he will surprise Ran in front of her house. Ran's happiness was the last thing on his mind before he lost consciousness
...
"What do you think Shinichi will do?" Agasa asked Ai as they look towards Shinichi's house. Ai looked at the Professor and smirked.
"Knowing that fool, he'll take the pill despite its consequences just to finally be with her." Agasa looked towards Shinichi's house again before asking Ai whether what was written on the note was actually true.
"Sacrifices must be made. The choice is up to him. He either chooses his original body back, only for its health to start deteriorating after three years or he can continue to live by her side as Conan as long as life allows them."
Before Agasa could ask anymore question, the two of them saw a figure by Shinichi's window. It was Shinichi himself with his grown body. Haibara smiled sadly before turning away.
You are a foolish detective. Good bye, Conan Edogawa.
~FIN~