According to HMV, the DVD & Blu-ray volume 7 of Ookiku Furikabutte ~Natsu no Taikai Hen~ will include an unaired episode 12.5. Director Mizushima posted in his blog that the special was made because they couldn't put the original manga episodes happened between the episode 12 and 13 within the TV series.

Pokemon are peculiar creatures with a vast array of different abilities and appearances; many people, known as Pokemon trainers, capture and train them, often with the intent of battling others. Young Satoshi has not only dreamed of becoming a Pokemon trainer but also a "Pokemon Master," and on the arrival of his 10th birthday, he finally has a chance to make that dream a reality. Unfortunately for him, all three Pokemon available to beginning trainers have already been claimed and only Pikachu, a rebellious Electric type Pokemon, remains. However, this chance encounter would mark the start of a lifelong friendship and an epic adventure! Setting off on a journey to become the very best, Satoshi and Pikachu travel across beautiful, sprawling regions with their friends Kasumi, a Water type trainer, and Takeshi, a Rock type trainer. But danger lurks around every corner. The infamous Team Rocket is always nearby, seeking to steal powerful Pokemon through nefarious schemes. It'll be up to Satoshi and his friends to thwart their efforts as he also strives to earn the eight Pokemon Gym Badges he'll need to challenge the Pokemon League, and eventually claim the title of Pokemon Master. [Written by MAL Rewrite]

In the future, an organization called the GGP has taken control of the world. Rin Ogata was a promising up-and-coming ballet dancer, but suffered a serious injury while dancing and decided to quit. Years later in college, she comes across a club building and soon finds herself intrigued by a transforming motorcycle-like robotic vehicle called a "Rideback". She soon finds that her unique ballet skills with balance and finesse make her a born natural on a Rideback. However, those same skills also get her into serious trouble with the government. (Source: Wikipedia)

The story takes place in a time when the world is in the pits of despair after being ravaged by a poison called "shinobi" that came to earth via an asteroid. The people run away to the place called "Kairiku," a place that used to be the bottom of the ocean. Here, the people try and protect themselves against the effects of the poison. In such a world, there was a single light of hope. The "Holy Weapon Giga Road" and the legendary warrior "Yagyuu" are needed in order to counter the effects of the poison. In order to save the world, Ame, a princess of a fallen nation, stands up in order to save the world. (Source: ANN)