Sonic the Hedgehog 4: Episode I is a 2010 side-scrolling platform game developed by Dimps, with assistance from Sonic Team, and published by Sega. It is part of the Sonic the Hedgehog series and acts as a sequel to Sonic & Knuckles (1994). Episode I follows Sonic as he sets out to stop Doctor Eggman, who has returned following his defeat in Sonic & Knuckles. The game returns to the Sega Genesis style of Sonic gameplay, with movement restricted to a 2D plane. Like previous Sonic games, the player races through levels, collecting rings while rolling into a ball to attack enemies. The game also features special stages in which the player collects Chaos Emeralds and online leaderboards comparing level completion times and high scores. Development began in June 2009 and lasted a year and a half. The game was conceived as a smartphone-exclusive spin-off before becoming a multiplatform, mainline Sonic installment. It was designed to appeal to both older Sonic fans who played the Genesis games and newer ones who played later games like Sonic Unleashed (2008). As a continuation of the classic Sonic titles, Episode I features no voice acting, a simple control scheme, level design emphasizing platforming and momentum-based gameplay, and no player characters besides Sonic himself; however, it incorporates Sonic's design and homing attack ability from Sonic Adventure (1998). Producer Takashi Iizuka and composer Jun Senoue were the only Sonic 4 developers who contributed to the Genesis games.
Episode I was released worldwide in October 2010 as a downloadable game for iOS, PlayStation 3, Wii, and Xbox 360. A release for Windows Phone followed in June 2011, for Windows and Android in January 2012, for BlackBerry Tablet OS in July 2012, and for Ouya in July 2013. The game received moderately positive reviews and sold over a million copies in under a year. Critics described Episode I as a satisfying return to classic Sonic gameplay and praised the sense of nostalgia. Criticism was directed at its physics engine, considered inferior to that of the Genesis games, and its short length. Episode I was envisioned as the first of an episodic video game trilogy. Sonic the Hedgehog 4: Episode II was released in May 2012, while Episode III was never made.