Sonic the Hedgehog is the first game of the saga of SEGA's famous spiky hero released in 1991 for Mega Drive/Genesis – and the company's original standard-bearer thanks to being a well-rounded game in all sort of ways.
This port isn't a simple replica run over an emulator. In fact the original code has been modified to offer substantial upgrades in the Android adaptation, like a panoramic resolution that doesn't stretch the image or fudge the aspect ratio. Plus it's got the option to unlock Tails and Knuckles in the first installment, something seen only in hacked versions of the Sonic & Knuckles combo, a cartridge that had an extra expansion slot to hook up the other 16-bit games from the saga.
Beyond that, the game is practically identical to the original: you've got to trek through eight worlds made up of three levels each in your attempt to get back the jewels from the chaotic clutches of Dr. Eggman/Robotnik. Warning: you may well tear up a little explaining this premise decades later to a new generation of gamers coming at this classic for the first time.
The game makes up part of the so-called SEGA Forever line, a series of games for mobile devices officially released by the company that are both identical in gameplay to the originals and offer the chance to customize the touch controls and even save your progress at any time. All are free-to-play, meaning you'll simply see a few ads in the start menu. Once you start playing there are no banners or anything of the sort to slow down your experience.