Sonic the Hedgehog 4: Episode I is a 2010 platform game developed by Dimps, with assistance from Sonic Team, and published by Sega. It is a sequel to Sonic & Knuckles (1994), following Sonic as he sets out to stop a returning Doctor Eggman. Like the Sonic the Hedgehog games released for the Sega Genesis, Episode I features side-scrolling gameplay, with movement restricted to a 2D plane. 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 May 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. As a continuation of the Genesis Sonic games, Episode I features a simple control scheme, no voice acting, level design emphasizing platforming and momentum-based gameplay, and no player characters besides Sonic; however, it incorporates Sonic's design and abilities from later games like Sonic Adventure (1998). It was designed to appeal to both older Sonic fans who played the Genesis games and newer ones who played more recent ones like Sonic Unleashed (2008). Producer Takashi Iizuka and composer Jun Senoue were the only Sonic 4 developers who contributed to the Genesis games. Episode I was released in October 2010 as a downloadable game for iOS, PlayStation 3, Wii, and Xbox 360. Versions for Windows Phone, Windows, Android, and BlackBerry Tablet OS followed throughout 2011 and 2012. The game received moderately positive reviews and sold more than one million copies within 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 planned as the first episode in a trilogy; Episode II was released in May 2012, while Episode III was never made.