Sonic the Hedgehog (ソニック・ザ・ヘッジホッグ Sonikku za Hejjihoggu?) is a platform video game developed by Sonic Team and published by Sega for the Sega Mega Drive/Genesis. First released in North America, Europe, and Australia on June 23, 1991, the game is the first installment in the Sonic the Hedgehog series, chronicling the adventures of the titular character in his quest to defeat the series' antagonist Dr. Robotnik. The game's story focuses on Sonic's efforts to stop Dr. Robotnik's plans for world domination, release the animals Dr. Robotnik has trapped, and collect six magical emeralds known as the Chaos Emeralds.