At the Shabboneau Castle which is situated on the northern edge of Camphrier Town, the Castle Master gazes upon a painting of the jewel Pokémon, Diancie, and marvels at its astounding beauty. He believes that its pink shiny body is indisputably the most dazzling thing in the world, and yearns to gain possession of it. Overwhelmed with craving, he rolls around on the floor, and thinks it will suffice even if he only gets to see it in person for once. At that moment, his team of servants arrives to greet him, and gets ready to set out again to continue their search for Diancie. The moustached man thanks them for their hard work, and reminds them that countless millionaires in the world have been captivated by Diancie's beauty and are all out there to seek it. He urges his servants not to let others beat them to the quest, and promptly dispatches them from the Castle.

Evening has fallen in Kalos. After a day of travelling, Korrina, Gurkinn and Diantha have settled down somewhere in the mountains where they can conceal themselves. Diantha and Korrina sit around a fire to have their dinner, and the young Shalour Gymleader asks if she really has the chance to become the Champion someday. Diantha replies that it is certainly possible, and quotes an example from a distant region, where a man took the Champion title after being a Gymleader for several years. Korrina feels fascinated by the story, but thinks she still has a long way to go, since she must at least be strong enough to beat the powerful Elite 4 first. She then gets curious about the personalities of the Elite 4, and begs Diantha to tell her about them. She wagers that they must be good friends since they guard the League together, and Diantha admits with a grin that they are indeed close.

Korrina first asks about Wikstrom, calling him the armor guy who wields steel types, and Diantha giggles that although he is a mature man, he can be quite stubborn sometimes. For Drasna, whom Korrina calls the amiable old dragon lady, Diantha thinks she gives a good description, and says the dragon type expert is an extremely friendly person who always wears a smile on her face. For Siebold, Korrina refers to him as the water type guy with scary eyes, and Diantha laughs at her depiction, revealing that he is actually a top cook who loves to entertain people.

Korrina finds it amazing that everyone in the team is so different, and wonders about Malva, the famous TV reporter. The thought of her causes Diantha to pause a little, and she thinks she will describe her as a woman of fire who is shrouded in mystery. Korrina gets the impression that Diantha is at odds with Malva, and Diantha quickly denies. She feels embarrassed that her facial expression gave off such a wrong message, and laughs that she fails as an actress. The truth is, she is least associated with Malva, since the woman never once joined the private meetings and practice sessions she holds with the other three. She believes that Malva is a simply a busy very person, but gives people the feeling that she is hiding a lot of secrets.

Korrina finishes up her meal, and decides to go see what her grandfather is up to. Diantha straightens herself up as well, and spots a few Carbinks hurrying out from a nearby cave entrance. She figures that they must have been scared off by some wild Pokémon they are frightful of, and follows Korrina back into the tent without giving much thought to it. Inside, Gurkinn wears a pair of special spectacles as he polishes Diantha's Mega Pendant, and Korrina wonders what he is doing. Gurkinn explains that he is performing maintenance, and says both the Mega Glove and Mega Pendant must be kept clean and pure at all times since they are the gears that hold the Keystones. He states that the same goes for the Mega Ring and every other gear he has created for the purpose, and Korrina decides to ask something which has been on her mind for a while. She wishes to know why Gurkinn doesn't simply create a standardized gear for every Successor, and Gurkinn laughs that she really has an inquisitive mind. The Mega Evolution Guru explains that the Keystone is an artifact which draws out the inner qualities of a trainer, and in order to facilitate this process and maximize its potential, the gear that bears the Stone must fit the style and personality of its user. Korrina thinks the Keystone is really a mysterious item, and Gurkinn grins that Mega Evolution itself is mysterious, which also applies to all the Pokémon who are capable of it.

After putting their respective gears back on, the three step out from the tent, and Gurkinn makes his way over to the nearby cave entrance. He muses to himself that one particular Pokémon living inside is also capable of Mega Evolution, and Diantha, aware of her teacher's interest in the cave, tells him about the Carbinks she saw earlier. Gurkinn gets a little bit worried after hearing Diantha's description, and reveals that he has actually chosen the location to camp out due to a concern he has. He understands that they must come up with a strategy about the 'Cocoon' as soon as possible, but hopes that Diantha and Korrina wouldn't mind checking out the cave with him first, even just for the sake of a peace of mind.

Gurkinn, Korrina and Diantha start making their way into the cave, and the girls are surprised that it is unexpectedly steep. Diantha says it feels more like an entrance to the hot and high-pressurized underground, and the group soon sees a horde of fainted Carbinks. Diantha is astonished that so many have fainted, and Korrina gasps that they have brutally attacked. She wonders who would have done this, and Gurkinn postulates that someone is trying to capture and sell them for money, since Carbinks are known as the jewel Pokémon and their shiny bodies are said to worth a lot.

At that moment, a voice comes on to say that Gurkinn made a wrong guess, and the three look ahead to see a thief who has concealed his face with a mask. The thief carries a feminine Pokémon whose body is made of pink jewels, and Gurkinn instantly recognizes it as Diancie. The thief is surprised that Gurkinn knows about the 'Pink Princess', and Gurkinn figures that the Carbinks must have been intentionally attacked to lure Diancie out. The man says he is simply doing his job as a jewel thief, and explains that his current client is willing a pay a lot for Diancie. He speaks of his strategy in disguising as a servant for a fallen aristocrat who possesses old texts pertaining to Diancie, and reveals how he secretly used the information for his own purpose to locate this cave.

Korrina finds herself confused about Diancie and the term 'Pink Princess', and Gurkinn explains that Diancie is in truth a mutated Carbink, whose rate and mechanism of transformation remains a mystery. He states that Diancie is said to live in caves where many Carbinks inhabit, and has earned the name 'Pink Princess' due to its beautiful pink and shiny appearance.

The thief hopes that the group is not trying to interfere since he really needs to bring Diancie back to his client, but Korrina wouldn't let him off easy, and the thief decides to send out three Binacles to fight. Korrina, Diantha and Gurkinn call forth Lucario, Gardevoir and Heracross in response, and promptly engage in a close-up combat with the barnacle Pokémon. However, Korrina soon sees that the two heads of each Binacle are capable of fighting independently, and feels troubled since they are essentially dealing with six opponents.

Gurkinn whispers under his breath, and wants Diantha and Korrina to lure the enemy outside the cave. Korrina fails to understand why, but Diantha believes that Gurkinn has his reasons and tells Korrina to play along. Lucario, Gardevoir and Heracross start pretending to be overpowered by the Binacles, and the thief grabs the chance to run past the group. He quickly dashes towards the cave entrance, and Gurkinn, Korrina and Diantha proceed to Mega Evolve their Pokémon before giving chase. Once they are outside, the group resumes their attack, and forces the thief to flee towards a nearby moon-lit hilltop.

As the thief steps into the moon light, Diancie begins to react to the full moon in the sky, and Gurkinn tells the girls to keep their eyes open. In the next instant, Diancie glows intensely, and a powerful beam of light suddenly strikes down from the sky, hitting the thief directly and tearing his mask and some of his clothes apart. The thief tumbles to the ground in astonishment as he is forced to let go of Diancie, and Gurkinn explains that it is Moonblast, an attack which calls upon the powers of the moon.

The thief exclaims that the fallen aristocrat's files did not mention such a move, and at that moment, the Shabboneau Castle Master arrives with his other servants in a car, furious to hear the term 'fallen aristocrat'. The moustached man reveals that he has actually been suspicious for some time, since some of his old texts about Diancie mysteriously disappeared after he hired the guy. Coupled with the strange fact that the guy always chooses to work alone on his quests, he deliberately followed him, and now finally confirms the fact that he wants Diancie for his own. Gurkinn explains that the man is merely a jewel thief who is hired by a client to find Diancie, and the Castle Master deems his actions unforgivable, especially the 'fallen aristocrat' part. He starts kicking at the man's face, and asks his servants to inform the police.

Just then, the moon gets shrouded by some thick clouds moving by, and the thief sees his chance. He thinks the Castle Owner should thank him since he gave him a chance to finally lay eyes on Diancie, and his words cause the moustached man to fume with rage. The thief laughs that with the moon hidden away, Diancie cannot use Moonblast, and calls forward his Binacles to attack. However, Diancie creates a globe of swirling energy between its palms, and without warning, fires off a barrage of hard diamonds to knock back the thief and his Binacles.

The Castle Owner exclaims at the beauty of the attack, and Gurkinn explains that it is called Diamond Storm, a move formed by rapidly compressing the carbon in the air to create diamonds. Diantha recalls to have heard of it as well, and says it raises the user's Defense in addition to inflicting damage on the opponent. Gurkinn turns to the thief and wonders what he plans to do now that his Pokémon have all fainted. The thief knows he has no choice but to retreat, and turns around to flee as Mega Heracross chases him off with a Bullet Seed.

Later, the jewel thief slumps in defeat and returns to where he is supposed to meet his client, who turns out to be Malva. The thief offers her a handful of big diamonds which Diancie created, and hopes that they would make up for his failure to secure the Pokémon. Malva sees that the diamonds are quite remarkable, and the thief wonders if it means he still gets some kind of reward for his work. Malva assures him that he will be rewarded, and without warning orders Pyroar to scorch him with a Flamethrower. The man screams out in pain as he runs around in flames, and Malva finds it amusing that there are people who are shameless enough to ask for rewards despite having failed their mission. Taking another look at the diamonds, she thinks they are beautiful nevertheless, and decides to keep them for herself.

As the sun begins to rise, the Shabboneau Castle Owner breaks into tears of joy, and says there are no more regrets in his life now that he has finally witnessed the beauty of Diancie with his own eyes and even got to see its amazing Moonblast and Diamond Storm. He makes up his mind not to search for it anymore and decides to destroy the remaining old texts he has, so as to avoid bad people from exploiting the information to hurt Diancie. Gurkinn thinks Diancie will be very thankful for his decision, and the moustached man bids the jewel Pokémon farewell as he gets ready to return to the Shabboneau Castle.

Korrina, Diantha and Gurkinn wave the Castle Owner goodbye, and to Korrina's surprise, Diancie suddenly places its hands over the Keystone on her Mega Glove. It then happily leaps over to touch Gurkinn's Keystone followed by Diantha's, and Korrina wonders what it is doing. Gurkinn explains that Diancie is actually a Pokémon who is capable of Mega Evolution, and thinks this is its way of connecting to them. In fact, he reveals that his true concern has always been towards people who are after this ability of Diancie's, and not those who are merely after its beauty and money worth. Now that this cave's secret has been discovered, he believes that Diancie must find somewhere else to settle down with the Carbinks.

And so, the Royal Pink Princess nods goodbye to the three humans, and sets out to search for another residence with its companions.

Later that day, at the Battle Chateau, Cassius informs the Vaniville kids that it is time to go and tells them to stay low and hidden in their seats as he drives. Trevor wonders where his assistants are, and Cassius replies that they are already waiting at the backyard. The van slowly enters the Chateau through its front door, and the butler expresses his gratitude to Cassius, stating that their many guests with noble ranks can finally use the 'Tray' and 'Battle Box' functions in their next visit. Cassius grins that he is only doing his job, and wants to know if it is really alright that he borrows the craft. The butler assures him that he is free to use it as he wishes, and Tierno, confused by what he has heard, whispers to Trevor and asks if he knows what is going on. Unfortunately, Trevor has no answer to his question, and the kids are left wondering what will happen as Cassius drives on.

Soon, the van arrives at the backyard, and Cassius instructs the young ones to disembark. To their surprise, they see a helicopter shaped like a Flygon waiting for them, and Cassius's team of punks swiftly help them get on. While the punks stay behind, Cassius climbs into the pilot seat, and in no time, the helicopter goes airborne.

A while later, up in the air, Trevor is still recovering from his shock at the sudden turn of events, and requests to know why they have switched from travelling on land to taking the aerial route. Cassius explains that in order to reach Cyllage City, they must go through the Connecting Cave, which is dark and inhabited by Zubats. He thinks it is wiser to avoid such shady areas where someone might ambush them, and the kids see that he has a point. Cassius wishes to talk about Clemont, and says he finally managed to remember who he is. Apparently, the boy is really the top inventor that he claims to be, and is also the Gymleader of Lumiose City. In light of the recent blackout in Lumiose which affects the power supply to the Prism Tower, he believes that Clemont must have gone to Cyllage to look for solutions, and encountered Y's mother by chance. Trevor realizes that Elec should be acquainted with Clemont since it used to be at the Prism Tower, and the Manectric nods with a sad face. Tierno could see that it is worried about the boy, and Trevor assures it that everything will be alright.

Suddenly, Cassius notices a few bright spots on the radar, and states that something seems to be approaching their craft. The number of spots rapidly increases, and Cassius counts with astonishment that there are at least fifteen of them. At that moment, something glides past the helicopter's side window, and Shauna exclaims in recognition that it is a flying suit just like Y's. In a matter of seconds, Sky Trainers commanding Vivillons materialize from every direction, and the Vaniville group soon finds themselves surrounded in midair…