The Japanese set revolves around the legendary Pokémon Dialga, Palkia and Darkrai and is a nod to their roles in the 10th Pokémon movie, The Rise of Darkrai, which was released just over a week after the release of the expansion.
Because of the close ties to the movie, Nintendo once again decided to delay cards to coincide with the English movie release, in a similar fashion to EX Deoxys. However, instead of delaying the entire set, only specific cards were removed. To fill the gap, several cards from Moonlit Pursuit/Dawn Dash were added, meaning they saw an almost simultaneous release to the ones in Japan.
As the Japanese half decks focused on Dialga and Palkia, their removal from the English set required the deck kit from Moonlit Pursuit/Dawn Dash to provide the theme deck material, hence their inclusion in the set.
Secret Wonders expands on the theme of Pokémon with held items. Instead of berries as with cards in Mysterious Treasures, they hold evolution items. Once again, there are only three cards to feature this and they have no collection number.
In an unusual move by Nintendo, several alternate holographic cards were excluded from being available in booster packs. Instead, they were reserved for special blister packs that were intended to be released simultaneously with the launch of Secret Wonders. Many of these packs were released before the launch of the set however. The reserved alternate holographic cards are Breloom, Flaaffy, Quagsire, Bulbasaur, Carvanha, Charmander, Shellos East Sea, Shellos West Sea, and Squirtle.