The game is split into several themed levels, some of which are based on locations from past games, such as Green Hill Zone. Players must complete each level while fighting waves of enemies, and (with the exception of Classic Sonic) seamlessly shift from side-scrolling to third-person perspectives. Stages are filled with features such as spring boards, rails, and boost pads, and obstacles such as spikes, bottomless pits, and robots. Scattered around levels are golden rings, which serve as a form of health: rings protect players from damage caused by robots or other hazards, though they lose their rings upon being hit; being hit with no rings kills the player. The game does not use a traditional life system, instead deducting from the player's score depending on the number of deaths. Players are given a grade dependent on their performance; an "S" rank is the best and a "C" is the worst. Certain levels feature a boss, which must be defeated in order to proceed. Bonus levels can be unlocked by collecting Red Rings hidden in most stages and players may occasionally receive SOS Missions, tasking them with replaying levels with a specific objective.
Plot
Doctor Eggman attacks a city, and Sonic sets out to stop him. However, upon arriving, he is attacked by Infinite, a mercenary transformed by the power of the Phantom Ruby from Sonic Mania. Infinite defeats Sonic, and he is taken prisoner aboard the Death Egg. Without Sonic, the land is left without a defender, and Eggman, with the help of his robots, Infinite, Shadow the Hedgehog, Chaos, Zavok, and Metal Sonic, manages to conquer the world within six months. Knuckles the Echidna forms a resistance force with Tails, the Chaotix, Amy Rose, Silver the Hedgehog, Rouge the Bat and a survivor of Eggman's attack whom they nickname the Rookie. Knuckles discovers Sonic's whereabouts, and the Rookie steals a space shuttle, breaks into the Death Egg, and rescues Sonic. Back in the city, Tails is attacked by Chaos, but is saved by Classic Sonic (who ended up in Modern Sonic's world following the events of Sonic Mania). The two set out to spy on Eggman, learning of the Phantom Ruby and that Eggman has a plan to completely destroy the Resistance which will take effect in three days.
Meanwhile, Sonic encounters Shadow, who reveals that the Shadow following Eggman is a fake virtual reality copy created by Infinite, as are the other villains serving him. Tails infiltrates Eggman's computer network and finds the Phantom Ruby's weakness: it is directly powered by the Death Egg's core, and without a power source it is useless. While the Rookie stages a diversion, Sonic and Classic Sonic destroy the Death Egg. Believing they have the upper hand, the Resistance attacks Eggman's capital Metropolis. However, Eggman has a hidden backup power source under Metropolis, and as Sonic and company make their final charge against the mountaintop Eggman Empire Fortress, he makes Infinite create a massive sun over the planet so that it will crash down and destroy the entire Resistance. Thinking fast, the Rookie uses a prototype Phantom Ruby they had recovered to dispel the virtual sun, saving everybody. Sonic and the Rookie then battle Infinite and defeat him. Eggman unveils yet another backup plan, moving the Phantom Ruby into one of his Death Egg Robots and fighting Sonic and his friends directly. Working together, Sonic, Classic Sonic, and the Rookie destroy Eggman's robot and defeat him. With the Phantom Ruby neutralized, all the copies serving Eggman's army vanish, Classic Sonic returns home, the Resistance disbands, and Sonic and his friends set out to rebuild the world while the Rookie leaves to find their own way.
Reception
Sonic Forces received "mixed or average" reviews, according to review aggregator Metacritic. In the United Kingdom, the game debuted in fifth place on their all-formats chart. Sega stated in a financial report in February 2018 that the game "performed strongly".
Writing for Polygon, Jeremy Parish praised the game's visuals and character customization, but criticized its repetitive boss fights and argued the game did not advance the series' design. Parish disliked the ranking system, which rewards players for completing stages quickly rather than exploring them for collectibles, and dismissed the tag team sections as "under-developed" because players would prefer to use Modern Sonic whenever the Avatar was not required. The four reviewers of Famitsu were more positive, praising the character creation system, story, and soundtrack. Heidi Kemps, reviewing Forces for IGN, faulted elements of the game's level design, indicating that "unfair-feeling traps" recurred intermittently and that the levels were relatively short and underdeveloped. In contrast to Parish, Kemps enjoyed the "quite strong" bosses, particularly one fought "among the winding coils of a giant snake," although she wished they were more plentiful. Kemps cited the game's "goofy charm" as preventing her from being "too disappointed" by it.
Kotaku's Heather Alexandra wrote that Modern Sonic's gameplay was "visually dynamic" and "the most fun", but heavily automated and shallow, whereas Classic Sonic delivered a solid but unremarkable platforming experience that compared unfavorably to the more inventive Sonic Mania. Alexandra panned the Avatar stages as "disjointed and confused" due to a lack of flow and an excessive reliance on trial and error. Overall, she described Forces as fun despite a lack of polish and complimented its "infectious energy and excitement". Game Informer's Brian Shea similarly regarded the game as a "fun adventure" and considered its controls and level design to be the best of any 3D Sonic entry, but concluded: "3D Sonic games still aren't to where they should be after such a long time of iteration and experimentation". Shea called Classic Sonic the worst of the three playable characters due to poor physics that failed to emulate those of the original 2D Sonic games. Ben "Yahtzee" Croshaw of The Escapist panned it for not exploring the potential of its ideas and lambasted the story for its attempt at a more mature tone. He later ranked it the second worst game of 2017. Several critics considered Sonic Forces a disappointment in the wake of the positively received Mania.
John Linneman of Eurogamer found that Forces was well-optimized for PlayStation 4, with no noticeable improvement when played on a PlayStation 4 Pro, whereas the Xbox One version was "slightly more responsive" but marred by screen tearing and inferior resolution. He felt that playing on Xbox One X provided the best overall experience, albeit with some "unwelcome inconsistency" caused by occasional changes in resolution, but that the Switch version had worse technical performance than Sonic Lost World (2013) on Wii U, which featured graphics of similar quality running at twice the framerate. The Windows version received criticism for problems related to its uneven performance and frequent crashes; Sega released a patch on launch day that fixed some of the problems.