After viewing a news report of a Shiny Gyrados sighting at a lake in the Johto region, the player sets off to check the local lake for anything.
You eavesdrop a conversation of Prof. Rowan and his assistant. After they're done, they leave, also leaving a briefcase behind.
The two are attacked by a wild Starly, there, you check the briefcase and you choose your Sinnoh starter. You battle the Starly and (usually) win.
The Prof. Assistant takes the briefcase, then realizes the Pokemon has a strong bond. He gives you and your rival a Pokedex and you set off.
As for Platinum, the storyline is not so different. in the beginning, you do the same exact thing in the beginning of D/P. The middle and end are much different.
Game Information
Release Dates
Diamond and Pearl
Japan: September 28, 2006
North America: April 22, 2007
Australia: June 21,2007
Europe: July 27, 2007
Platinum
Japan: September 13, 2008 Isn't released in other countries.
