Unlike Pokémon Yellow, Crystal and Emerald, Pokemon Platinum is very different from the games it's a remake of.
Just like any other Pokemon game, you're a kid who wants to be a Pokémon Master. You live in a small town called Twinleaf Town and your friend also wants to be a Pokémon Master. The two of you decide to go to Professor Rowan in Sandgem Town to get a Pokémon. Rowan finds you before you run into the grass (where you find wild Pokémon that will attack you) and thinks the two of you are worthy of having your own Pokémon. You will be given a choice of Turtwig, Chimchar or Piplup.
The Graphics haven't changed much. It's still 3D graphics but you move in two dimensions (excluding Distortion World). Most of the Sprites for the Pokémon have been changed and now the sprites for the Major trainers in the game will have an animation (your friend acts like an idiot). Some of the attack animations have been changed slightly as well. The audio is the same but there is some new music that has been added into Platinum to go with the new features.
There has been a lot of changes in Platinum from Diamond and Pearl, The Sinnoh Pokédex has been increased from 150 to 210, and The GTS has been replaced with the global Terminal which has more features as well as the online trading. Speaking of online a new feature is the Wi-Fi Plaza. Here you can play mini-games, chat or just play with animated stamps with people from all over the world. There are now 3 new move tutors who teach your Pokémon some special moves in exchange for shards which you find throughout the game.
The storyline has a huge change and I think that it's better. Team Galactic is still the main villain team but you're not the only one after them, you eventually meet up with Looker, a spy for the police force and for the rest of the game he pops up here and there to help you in many ways. The catchable Pokémon is different in Platinum too. In Diamond and Pearl you get the chance to catch Dialga or Palkia at Level 47 at the top of the Spear Pillar in Platinum you get the chance catch Giratina after going into a portal to the Distortion World at level 47 and both Dialga and Palkia at Level 70 later in the game. Rotom now has 5 new formes if you obtain an event item and Shaymin gets a new forme with an event item.
The goal of the game is the same obtain all 8 gym badges and beat the Elite Four and the Champion in the Pokémon League. After that you get the National Pokédex (all 493 Pokémon) and you can access more areas, more Pokémon and more features including a brand new Battle Frontier which has 5 batting facilities each with its own set of rules. It's a fun part of the game especially the Battle Arcade where each battle has a random effect. The new features are great!
Pokémon Platinum is a HUGE improvement of Diamond and Pearl and I highly recommend this game to anyone who has played Pokémon games before or for those who have never played one before. This is the best Pokemon remake yet thanks to all the new features. Pokémon Platinum is simply the best Pokémon game yet.