When the player and Cain first reach Agate City, they will find Aya and Hardy asleep in the middle of the streets. Shortly later, the player will encounter Terra and is brought to the circus by her.
In the circus backstage, the player will meet the other two circus leaders, Samson and Ciel. They suggest that the player should go and look for Hardy while Cain and them will look after Aya. Before leaving, Samson tells the player that although they are gym leaders, they won't accept battles unless the challenger has a battle pass.
After finding Hardy and obtaining the Battle Pass - Fury, the player can return to the circus and Samson will accept the battle. Defeat Samson and grant access to using HM Surf outside of battle.
Later on, after Cal reveals that Terra is part of Team Meteor, the player returns to the circus. When the player goes back to the circus backstage, Terra will then be cornered and escapes into a computer. The player is then forced to hunt her down and fight her in the Glitch World. After the player defeats her and leaves the glitch world, Samson turns off and restarts the computer she was in while she was inside. The current state of Terra still remains unknown.
Later on, after helping Titania in the Water Treatment Center, the player returns to the circus and Ciel will give the player the final battle pass. Defeat Ciel and grant access to using HM Fly outside of battle.
Due to the sleeping spell in Agate City, you will not be able to access places before Agate Circus until after you activate Fly. However, there are some stands in the circus that are useful, which includes:
A Name Rater
A Move Re-learner
An Ability Capsule Seller (you need some shards to get them)
On the second floor of the Agate Circus's checkpoint, there is a clown. He gives you a riddle, and you must present the Pokemon matching his description. This Pokemon is Stunfisk. He will trade you the Stunfisk for a Vulpix.
(Vulpix was previously available earlier in the game in the salon in Reborn City. However, it was moved here much later in the game.)
At the left checkpoint of the Agate Circus is an abandoned kid who wants specific items. His request is as follows: Cotton Candy, which can be purchased immediately by the east checkpoint for 5000 Poke; Air Balloon, which is found in the southern shops with a pink tent for 400 Poke; and a Poke Doll, received from getting a "Powerful!" from the High Striker.
Completion of the quest yields an Obsidia Department Sticker and the revelation that the kid was extorting you, as his parents were still alive.
The telecommunicators for Samson, Terra and Ciel are located in the backstage of the Circus. Although you do not battle Terra in the Circus, her telecommunicator is still located there.
You made it to the Agate Circus? Sounds like it's a mess up there... As you know, Agate Circus services three separate leaders. So Samson is first?
All right. He's fresh into the ring as far as leaders go, so I can't say too much... but I'll tell you what I know about the Big Top Arena. There's actually two main parts to it-- the first is probably Samson's play, the High Striker. The entire ground is made of panels that trigger it. They were originally set up for Terra, as any attack that hits the floor hard will activate it. But since fighting moves take strong power stances, they end up triggering the panels too. See if you can get a bonus from it as well!
Alternative, you might use Ciel's type advantage part of the field against him. Some flying types like Acrobatics and Bounce can make an extra splash around the Big Top with Ciel's aerial scaffolding. A few of those, and Samson should go down in no time!
Glad you picked up. I heard from Cal about what's going on. I never would have thought Terra of all people would be a Meteor... I mean, I never liked her anyway, so whatever. But goodness...
Anyway, I patched into her computer after I heard there was something up with it, and I can see it reporting her presence. ...I'm not going to question how she managed to enter a computer world at all; that girl already defies logic. I'm running an analysis now-- hold on, and I'll tell you what to expect.
The results are... strange. It looks like the computer environment is set up to simulate an old video game. In this game, attacks are physical or special based on what type they are. Plus, the Special Attack and Special Defense stats are treated as a single value-- it looks like it works off of which ever stat is higher.
Finally, the Dark, Steel and Fairy types seem to have been erased. This all sounds familiar, right? Keep these things in mind when you're in there. I know you can do what's necessary.
So you need to fight Ciel? Right. This won't be the first time you've fought on this field, so try to keep in mind everything you used before against Samson.
Unlike Samson's all-out offensive, Ciel plays more defensively. Many of her Pokemon have bulky set-ups, so they can be hard to power down. But Ciel is a performer at heart, and it shows in her battles, as she prefers to use dancing and sound-based moves. And being in this arena, it's the perfect place for those things to get her going.
If you can set up and knock her down before she even gets in the air, you should do well! But if she gets her act in the air, it's gonna be a long way down for you.
Outside the main circus tent, you can find a clown who will offer you a battle against one of ten monocoloured teams. This is a good area for raising your Pokémon, as you will be locked out of Reborn City, and you will earn a random shard each time he is defeated.