First of all, the characters. Kōki is Diamond. Jun is Pearl. Hikari... will she still be Hikari, or will she be named after the as-yet-unconfirmed third game? Hmm.
Diamond & Pearl, apparently, can be said to be more of a comedic duo than Pokémon trainers. They collide with Nanakamado in a crowd, and this incident will change their fate.
A letter, detailing what appears to be an escort. The goal: Mt. Tengan. The starting point: Kotobuki Mansion. Departure: 5PM, September 28. Compensation: ¥10,000,000. (That's a whole order of magnitude more money than the player character can have! ...)