Arceus

One night...
Ash was dreaming. He was dreaming that he was in a big cave in Mt. Coronet and all the strange plates he had gathered were surrounding him.
"What's going on?" He asked.
Suddenly the plates flew away and Ash chased after them. The plates came to some sort of ancient ruins.
"What is this place?" Ash asked.
Just then, the plates disappeared underground and a big shadow of light appeared in a flash. Ash woke up from his dream with a jolt.
"What was that about?" He asked out loud.
The next day Ash told the others about his dream.
"That's certainly a strange dream Ash." Dawn said. "It's almost like when we dreamed about Azelf, Mesprit and Uxie, but different."
"Yes and to be honest, I've had this dream ever since I've started collecting these strange plates. At first it was just one plate in front of me, and when I collected more of the plates they would appear in my dream." 
"Wow." Dawn said. "Then what are you going to do, Ash?"
"I'm thinking I should return to Mt. Coronet." Ash answered. "If my dreams keep showing me Mt. Coronet, then the answer might be there."
"But Ash, we're miles away from Mt. Coronet." Dawn said. "And it will take us a while to get back there again." 
"Not if we teleport there." Ash replied. "I can teleport us back afterward too."
"Really?" Dawn said. 
"Of course. I've practiced with my teleportation a lot so I can do it." Ash stated.
So the gang agreed to go to Mt. Coronet to figure out the meaning behind Ash's dreams. Tomo, Dawn and Brock stood close to him and then he used Teleport to get to Mt. Coronet. Thanks to Teleport the gang made it to Mt. Coronet in a second.
"That was amazing." Tomo said.
"That's what you call a shortcut." Brock said.
"I'll say." Dawn agreed. "Your powers never cease to amaze me Ash."
Ash took out all seventeen plates from his backpack.
"I can't believe you found all those plates at locations we visited in the past Ash." Dawn said.
"Same here." Ash replied.
"So what do we do now?" Brock asked.
Before Brock's question could be answered, the plates started glowing and floating into the air and then they zipped into the cave.
"What just happened?!" Dawn gasped.
"I don't know, but let's follow them." Ash said.
The gang followed the plates into Mt. Coronet. The plates flew deeper and deeper into the cave.
"Where are the plates going?" Tomo asked.
"Maybe they're going to the Spear Pillar." Brock guessed.
His theory turned out to be right because the plates did fly to the Spear Pillar.
"I wonder why the plates brought us here?" Dawn questioned out loud. "We aren't dealing with Dialga, Palkia and Giratina again, are we?"
"I don't think so Dawn." Ash said. "I mean the shadow from my dream resembled a Pokemon, but it wasn't shaped like Dialga, Palkia or Giratina."
"Then what was it?" Dawn asked.
Before Ash could reply, the plates glowed brighter than ever and then they disappeared under the ground.
"Hey, that's what happened in my dream." Ash said.
Before he could say anything else, a big orb of bright light came out of the ground and then it faded away to reveal an Arceus.
"I can't believe it. It's Arceus." Dawn said, surprised.
The seventeen plates circle around Arceus.
"The plates must have belonged to Arceus." Brock whispered.
Arceus looked at the gang, then he stared at Ash and silently roared.
"What's wrong with Arceus?" Dawn asked.
"I think he wants me to get closer to it." Ash said.
Before anyone could ask why, Ash bravely stepped forward towards Arceus. Arceus stared at him for a moment and then roared while glowing.
"Oh no." Dawn said.
She was worried Arceus was going to attack Ash. But Arceus didn't attack him. What actually happened was Ash started to glow.
"What's going on?" Dawn asked, worried.
"I might be wrong, but Arceus seems to be bonding with Ash." Brock said.
"You mean like how Ho-Oh bonded with Ash?" Tomo asked.
"Yes." Brock replied.
Tomo, Brock and Dawn kept silent while Ash knelled down on the ground. Arceus and Ash continued glowing some more and then it faded. Then Arceus roared and flew into another dimension.
"What happened Ash?" Dawn asked.
"You may not believe me Dawn, but Arceus bonded with me." Ash said. "What happened was that big orb of light was Arceus' egg. And the egg will only hatch if the seventeen plates were presented to it."
"Oh I get it." Brock said. "Since you found the plates, you helped Arceus hatch."
"Exactly." Ash said. "So out of gratitude Arceus has become my friend and it said if I need it's help it will come any time."
"Wow." Dawn said. "You really do have a way with legendary and mythical Pokemon Ash."
Ash used Teleport again so they could return to their previous location. 
So with the mystery behind the plates resolved, the gang could resume traveling to Lake Valor.