Sonic the Hedgehog, meet Sonic the Hedgehog.
The game takes off with Sonic celebrating his birthday when it's busted up by a being called the Time Eater who sends them through time holes and Sonic and Tails meet their younger selves from the Sega Genesis era. Then they run to the right like in all classic Sonic games.
The game has Sonic levels from both the earlier games and the newer ones: You can play as Classic Sonic in Green Hill Zone and Casino Night Zone (which happens to be one of my favourite levels in all Sonic games) and Modern Sonic in Emerald Coast Zone. There are eight zones in all. Boss battles take place in the 3DS version against Metal Sonic, Big Arm, Shadow, Silver and other rivals of Sonic's. Modern Sonic can use all his usual abilities like wall-jumps and grinding on rails while Classic Sonic can only do his jump and speed dash, a gesture towards the series' begginings.
What makes Sonic Generations such a great game is that instead of relying on just nostalgia to make it worthwhile, it takes the levels from older and newer Sonic games and recreates them nicely. There's plenty of replay value in mastering the levels, and while it lacks the Mega Drive version of the original game unlike the console versions, it's mostly a completely different game from the console versions and is well worth playing.