It's Sonic's birthday, and all his friends have thrown him a surprise party. Unfortunately, just as the celebration gets underway, a mysterious creature appears in the sky and pulls Sonic and his friends into strange interdimensional portals! This strange creature is wreaking havoc on the time-space continuum, and Sonic along with a younger version of himself end up at the crossroads of time. The two Sonics must work together to restore his history, save his friends, and fix the flow of time!

Sonic Generations is a fast-paced action platform game in the long-running Sonic the Hedgehog series. As part of Sega's 20th Anniversary celebration for the character, it revisits the history of the entire series with famous stages from all of the major titles over the last 20 years. The game is divided into three "eras", each with three stages, which each come from a different game starting from the original Sonic the Hedgehog all the way up to Sonic Colors, and including many of the highs and lows in between.

Each stage in the game comes in two flavors, or "Acts". Act 1 of each stage is played using classic Sonic, whose only abilities are to run, spin jump, and spindash. Act 1 stages are always entirely side-scrolling platform stages, much like the classic 2D titles. Act 2 stages, conversely, are played using Modern Sonic, who loses the spin dash but gains a homing attack and the ability to boost forward at extremely high speeds. Act 2 stages switch between 2D and 3D platform sections as the stage progresses. Both Sonics can earn and equip a wide variety of abilities such as quicker stopping, or new types of shields.

Each Era includes a one-on-one battle against one of Sonic's rivals, and culminates in a reimagined version of a classic boss battle. Clearing stages also opens up a huge number of challenge stages centered around specific side characters or gimmicks. Each main stage also has a set of collectable Red Star Rings to find. Collecting Red Rings, as well as completing challenges, will earn access to new and original music, as well as new and original concept art from across the series' history.