The game is based on the book and the two films of the same title. It follows protagonists Harry Potter, Ron Weasley and Hermione Granger who set out to destroy the seven Horcruxes, which will help them to defeat Lord Voldemort, the game's antagonist.
The Nintendo DS version is played in the puzzle based gameplay. The game shows a map of the area on the top screen, and a 3D top-down view of Harry and his nearby surroundings on the lower touchscreen. In the game the player controls Harry Potter with the stylus, moves him by dragging the stylus to the sides of the screen, and uses the stylus to equip spells and objects or attack enemies by tapping on them. In addition to the main character, Ron and/or Hermione are present to aide in combat. Although these characters are computer-controlled, they will attack the same enemy Harry targets. Additionally Ron and Hermione have their own abilities; Hermione can use the spell "Arresto Momento" which slows enemies down, and Ron uses a "Deluminator" which can extinguish lights. The game also features a multi-player mode where two players, using Wi-Fi connectivity, can play against each other with one player guiding Harry Potter to hunt for magic points, while the other controls three Dementors who try to hunt him down. In the game the player aims to get the highest score.
Source: Wikipedia, "Harry Potter and the Deathly Hallows: Part I (video game)", available under the CC-BY-SA License.