Sonic the Hedgehog 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.
Development of Sonic the Hedgehog began in 1990, when Sega ordered its AM8 development team to develop a game featuring a mascot for the company. After a hedgehog was decided on as the main character, the development group was renamed Sonic Team.
Sonic the Hedgehog received positive reviews from critics, who praised the game's visuals and its sense of speed. It was commercially successful, increasing the popularity of the Mega Drive/Genesis and establishing Sonic the Hedgehog as the company's mascot. Its success led to the development of subsequent games in the Sonic the Hedgehog series, as well as the creation of a media franchise of spin-off products featuring the character.
In an attempt to steal the six Chaos Emeralds and harness their power, the game's antagonist Dr. Ivo Robotnik ('Dr. Eggman' in the original Japanese release) has trapped the animal inhabitants of South Island in cybernetic shells and metal capsule prisons. The player controls the titular Sonic the Hedgehog, who aims to stop Robotnik's plans by freeing his animal friends and collecting all Chaos Emeralds himself.
Sonic the Hedgehog plays as a 2D side-scrolling platformer. The gameplay centers around Sonic's ability to run at high speed through levels that include springs, slopes, high falls, and loop-the-loops. The levels are populated with hazards in the form of robots (named 'badniks' in the Western game manuals) that Dr. Robotnik has trapped animals inside; destroying one frees the creature, but is not necessary to complete the game. The player must also avoid touching rows of sharp spikes, falling into bottomless pits, and being crushed by moving walls or platforms, as well as drowning, which can be circumvented by breathing air bubbles periodically released from vents. Sonic's main means of attack is the Spin Attack, in which he curls into a ball and rotates rapidly, damaging enemies and certain obstacles upon collision. This can be performed by jumping in the air or by pressing down on the D-Pad while moving on the ground.
Scattered around each level map are gold rings. Collecting 100 rings rewards the player with an extra life. Rings act as a layer of protection against hazards: if Sonic holds at least one ring when he collides with an enemy or dangerous obstacle, he will survive. However, all of the rings in his possession will be scattered; they will flicker and disappear in a few seconds if not picked up again. If he is hit without holding any rings, then he loses a life. Shields and temporary invincibility can also be collected to provide additional layers of protection, but certain hazards, such as drowning, being crushed, falling down a bottomless pit or running out of time, will cause Sonic to lose a life regardless of rings or other protection.
The game is split up into six zones—named Green Hill, Marble, Spring Yard, Labyrinth, Star Light, and Scrap Brain—each with its own visual style and enemies. Each zone is split into three acts. The player must navigate through each zone to progress. At the end of each zone's third act, the player confronts Dr. Robotnik, who pilots a vehicle, in a boss fight. After completing the sixth zone, the player continues directly to the 'Final Zone' for the last encounter with Robotnik. The player is given three lives—although powerups and rings give more—which are lost when Sonic collides with hazardous enemies or objects while having no rings, falls off-screen, or exceeds an act's ten-minute time limit. Lampposts that act as checkpoints allow Sonic to return to the most recently activated post when he loses a life. If Sonic loses a life as a result of time running out but has another life, the time will reset to 0:00 when he returns to the checkpoint. If all lives are lost at any point in the game, the 'Game Over' screen will appear, at which point the player can return to the beginning of the act with three lives if the player has any continues.
When Sonic reaches the end of Act 1 or Act 2 of a zone while holding at least 50 rings, a large ring appears through which he can jump to enter a 'Special Stage.' In the Special Stages, Sonic is continually curled up in his Spin Attack animation, and bounces off the bumpers and walls of a 360° rotating maze. In these levels, the player earns a continue with each 50 rings found, but the main goal is to obtain the Chaos Emerald at the end of the maze without colliding with any of the 'goal blocks' that instantly end the level. If the player collects all the Chaos Emeralds and completes the game, the most optimal ending sequence is shown. However, if not every Chaos Emerald is collected, a screen shows Robotnik taunting the player.
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