The story begins with Shadow infiltrating Eggman's latest base after hearing that he was working on a new ultimate weapon and swiftly takes down Eggman's new Jackal mercenaries, utterly humiliating their leader. A few months later Shadow is attacked by a monster wielding a ruby that can alter reality, the monster identifies himself as the leader of the Jackal squad now calling himself "Infinite".
Sometime later, Eggman's forces are attacking a city when Sonic arrives to stop the attack as usual but he's attacked by Infinite, who appears to control some of Sonic's past enemies. Sonic is defeated and presumed dead, allowing Eggman to conquer 99% of the world. 6 months later Sonic's friends form a resistance to wage war against the Eggman Empire. During the war Infinite kills a squad of soldiers; only one soldier survives, whom Infinite allows to flee in fear.
The survivor joins the resistance and is appointed as the new Rookie and is given a grappling hook and a "Wispon" weapon. Sonic is discovered to be alive so the resistance raids the Death Egg to rescue Sonic. During the raid Sonic defeats Zavok who then dissapears. Sonic escapes and runs into the Rookie, the two find a shuttle and flee the Death Egg.
Elsewhere Tails is attacked by Chaos, suddenly a wormhole opens from which a younger looking Sonic appears, he saves Tails from Chaos. Tails quickly identifies "Sonic" as a younger Sonic from an alternate timeline who was probably dragged to this dimension due to Infinite's reality warping powers. Tails and Classic Sonic team up to look for Infinite's weak point.
Silver briefly battles Infinite but is defeated, during the fight a replica of Infinite's ruby lands nearby. Sonic saves Silver and fights Infinite again, he takes the upper hand for a moment but Infinite's power again overwhelms Sonic. The next day Tails and Classic Sonic sneak up to Infinite and Eggman and learn that the ruby in Infinite's chest is the source of his power and is named the "Phantom Ruby". Eggman had produced countless replicas of the Ruby until he perfected the one Infinite is using. Eggman gloats that in 3 days he'll destroy the resistance. The Rookie investigates the area where Silver fought Infinite and finds a Phantom Ruby replica, then encounters Tails and Classic Sonic.
Sonic is attacked by "Shadow" when suddenly the second Shadow appears and defeats the impostor. Shadow explains that Infinite uses the Phantom Ruby to produce replicas of Sonic's enemies; no matter how many times they defeat the replicates, Infinite will keep producing more. Sonic, Classic Sonic, and the Rookie reunite and discuss their findings regarding Infinite's power. The Resistance launches an attack to recover Metropolis city but the mission fails when Infinite appears and uses the Phantom Ruby to eradicate most of the army. The Rookie battles Infinite, the two seem evenly matched until Infinite activates the Phantom Ruby, at that moment the Rookie's own Phantom Ruby neutralizes Infinite's power, forcing him to retreat.
The resistance comes to the conclusion that if they want to defeat Infinite they must destroy the Phantom Ruby, they discover that it's being powered by the Death Egg in scape so they destroy it. With the Ruby presumably shut down Sonic goes to Metropolis again but to his surprise, Infinite's Phantom Ruby is still fully functional. Eggman reveals that he has a backup power generator in his fortress. The resistance raids Eggman's fortress when Infinite uses the Phantom Ruby to project a large sun that will kill everyone as well as a massive army of replicas. Tails realizes that the Rookie's Phantom Ruby is their only chance to stop the virtual Sun. The Rookie rushes to erase the virtual Sun right before it crashes, however that also destroys the Phantom Ruby replica.
Projecting an entire sun weakened Infinite's Phantom Ruby, allowing Sonic and the Rookie to defeat Infinite once and for all. Infinite vanishes and Eggman appears with a massive Death Egg Robot holding the real Phantom Ruby; the one Infinite had was just another replica. Classic Sonic, Sonic, and the Rookie all combine their might to battle Eggman's mech and destroy it along with the Phantom Ruby, deleting all the replicas and putting an end to Eggman's dictatorship. With the Phantom Ruby gone, Classic Sonic returns to his proper dimension.
The war is over, the survivors are slowly repairing the world, and the Rookie decides to leave the resistance to go on their own adventures.
- Level design is very simplistic and rudimentary with frequent autopilot sections that require little player input and tend to be under 2 minutes long.
- It was discovered that only 3 people (Jyunpei Ootsu, Shoko Kamiya, and Yuya Setogawa) worked on the game's level design, two of which (Shoko Kamiya and Yuya Setogawa) have never worked on a Sonic game before.
- The special Episode Shadow DLC was heavily rushed. It only has 3 levels, two of which were recycled from the main game and Shadow plays just as a Sonic reskin.
- The plot is rushed and poorly written. The cutscenes are often just the characters talking and standing. The tone is very inconsistent, constantly changing between trying to be dark and not taking itself seriously. Cutscenes often abruptly end and the potentially exciting parts happen off-screen then are told in static dialogues rather than actually showing them to the player. It also has a fundamental conflict with the gameplay, in that it is rather hard to tell a story about a war in a game where the character has to solve all gameplay challenges by going very fast from one place to another. Some examples of poor writting include:
- The first cutscene makes it seem like Sonic is dead, but less than 5 minutes later Sonic is revealed to still be alive.
- Classic Sonic randomly appears, defeats a Chaos clone, then does nothing for the rest of the story.
- Characters constantly talk about big epic fights happening in the levels but none of that is ever shown, not even as background details.
- Aside from a few Death Egg Robots in the background of one zone and a flashback where some civilians die, it is never shown how Eggman conquering the world has affected anything.
- Sonic and the Avatar are sucked into a "void black hole" to a dimension where nothing exists at the end of one level, but the next level begins with them immediately escaping, with their whole time in the void only lasting 17 seconds. Given the area has a three-minute music track and Sonic acts like him and the Avatar have been in there for quite some time, it appears the level was cut.
- Infinite and Eggman both just disappear after their respective final boss fights and it's never shown what happened to them.
- Overuse of nostalgia just for the sake of fanservice, many of which were redundant because both Sonic Generations and Sonic Mania had already used Sonic nostalgia as a selling factor:
- Classic Sonic was shoehorned in and has no effect on the plot whatsoever as mentioned above. His gameplay also feels redundant because Modern Sonic and the Avatar also have plenty of 2D sections in their own levels.
- Maps like Green Hill and Chemical Plant are used yet again for no reason involving the story and are visited too many times.
- Other characters also appear but are never actually used for anything. For example, Chaos from Adventure 1 appears but is defeated by Classic Sonic in a cutscene; you never fight against him.
- Shadow (one of the antagonists that appears alongside the other villains) never gets fought ether, as he gets defeated by the real Shadow in a cutscene, again.
- Clunky physics with weird acceleration and little momentum that make precise platforming awkward and makes the characters feel too heavy, similar to Sonic Lost World and Sonic Coiors.
- You need a running start before making jumps otherwise you'll barely get any distance and miss your jumps; at the same time, if you jump while running fast, you'll jump too far and overshoot platforms.
- When running forward in 3D, you can hardly move left or right without side-stepping.
- Classic Sonic doesn't gain any speed when going down hills and will fail to go up a ramp unless he spin dashes beforehand.
- Although the Boost Formula from previous well-received Sonic games returns, it is a very watered-down version with some mechanics missing.
- Drifting, in particular, is missing, which means that sharp turns are near impossible to take without coming to a complete stop. One good example is in the level Metropolitan Highway there are sharp turns, so if you boost and try to lean to the left or right you will fall off the bridge.
- This, along with the large amount of on-rails sections, make the game feel even more autopilot.
- The boss fights are very hit or miss.
- Metal Sonic and Infinite's boss fights are near identical.
- As mentioned above, you never fight Chaos and Shadow as they are defeated in the cutscenes.
- Avatar bosses are made too easy with the Flamethrower, which can defeat the bosses in less than 1 minute.
- The Final boss's third phase is a repeat of Sonic Colors' final boss.
- Some quick time events have no penalty for failing to complete them. Quite literally, if you don't touch the controller at all during QTEs the game will just keep playing as if you had cleared them. Same happens for the "Double Boost" sequences where you can put down the controller and watch the game literally play itself for over 10 seconds.
- The character customization is rather limited, while you can choose gender, species, and coloring, you can only change minor facial features and give them clothing. You can't alter their size nor adjust body proportions, and the facial features have very limited options.
- Speaking of the clothing, there are many clothing items that are just recolored duplicates.
- Getting all the customization pieces requires a ton of grinding.
- The characters (especially Sonic and Tails) have been exaggerated badly in this game. Some examples:
- Earlier trailers suggested Sonic would be more serious and tough, as they show him clutching his right fist with an angry look on his face. It was even stated in-game that he was imprisoned and tortured for months after the first level, however he still has the same annoying trying-too-hard-to-be-90s-cool personality from recent Sonic games (though "6 months of torture" was a mistranslation from Japanese). In addition, he never shuts up about the power of friendship.
- Tails is made into a complete coward who always relies on Sonic, and can't even handle Chaos 0, even though he defeated Chaos 4 in Sonic Adventure.
- Infinite is severely underutilized, all he does is edgy talking and producing clones of previous villains. As a result of the rushed Episode Shadow DLC, his backstory is very weak and poorly written.
- Many characters have laughably bad lines like Tails' "True dat!", Sonic telling Classic Sonic "It's been generations since I've seen you!"
- The game throws tons of rings at you, but since the game has unlimited lives, they don't serve much of a purpose other than giving the player an additional hit point and increasing your score for S ranks.
- Much like Sonic Colors and Sonic Generations, the game can be completed in about 3 hours if you don't go after collectibles.
- Time trials can be very frustrating due to the awkward jumping physics and the unforgiving time limits.
- If you die during a SOS mission, you automatically fail the mission and can't try it again.
- Despite the clunky physics and short length, levels can be enjoyable, some highlights include Egg Gate, Capital City, and the Null Space level.
- It introduces the ability to create your own custom character to play as, referred to as the "Avatar". The Avatar can be either male or female and can be one of seven different species (Dog, Cat, Wolf, Rabbit, Bear, Bird, and Hedgehog), each having minor abilities in-game.
- Compared to the other playable characters, many critics and fans found the Modern Sonic levels to be more enjoyable .
- The Avatar Character plays similar to Modern Sonic but instead of boosting he/she uses a grappling hook and "Wispon" weapons. There are 7 different Wispons, each has a main attack and a powerup that can be used by collecting the corresponding Wisp capsule. There are also Wispons with special perks.
- There are over 700 different pieces of clothing that can be used to customize the Avatar's appearance, including outfits from other Sega characters and even a shirt with the infamous Sanic meme. One of the DLC is even a crossover with Persona 5 (due to Sega's purchase of the company Atlus).
- There are a few levels where both Sonic and the Avatar are controlled at the same time.
- In addition to the main levels, there are several mini levels you can unlock by collecting Red Rings. These bring mechanics that were previously used in Sonic Colors.
- While not the best, the soundtrack is very good despite heavy usage of synth and dubstep. Every Avatar level soundtrack has lyrics that symbolize what the Avatar is thinking at that moment.
- Despite only being used once in-game, Infinite's theme song is awesome (even though it tries way too hard to be edgy and had misleading lyrics).
- The background song that plays during the Eggman cutscenes really does add well to his intimidation.
- Fading World is a pretty cool song as well.
- While some characters like Sonic and Tails are poorly portrayed, most of the others were improved.
- After a few games of being portrayed as an irrational bully who's only interest is picking fights with Sonic, Shadow's personality in this game reverts back to his level-headed and heroic personality from previous games like Sonic Heroes and Sonic the Hedgehog 2006.
- After a while of being portrayed as an evil scientist who always came off as a joke, Dr. Eggman has gone back to his serious mad scientist personality from previous games, and goes back to his plan of conquering the world. The fact of how dark and serious he acts in the game can especially come off as quite jarring. He's also gone back to being the main antagonist again, and for a change, he isn't betrayed by the evil power he's trying to control.
- After some games and spin-off of him being portrayed as a generic idiot, Knuckles is more aligned with his previous personality from the Sonic Adventure series.
- Amy Rose's personality has drastically improved. Her personality portrayal from previous games due to Sega deciding to align her personality with her Sonic X counterpart consists of her obsessing over Sonic, being selfish and annoyingly overbearing, and just being quite cruel and physical. Her portrayal in this game is more aligned with Sonic Adventure's series portrayal.
- Great voice acting. Especially from Kirk Thornton as Shadow where his voice performance has largely improved since Sonic Boom, Travis Willingham for a quite spot-on impression of Dan Green, and Mike Pollock as Dr. Eggman for giving a more darker and serious performance compared to his previous roles as Dr. Eggman. Liam O'Brien was also great as Infinite.
- Completing missions and getting S Ranks in levels unlocks more Avatar outfit parts.
- You can play as other players' Avatars.
- Hunting collectibles and clearing missions does offer more challenge and adds flavor to the levels, you're also constantly rewarded with Avatar customization pieces for getting them.
Sonic Forces, known as "Project Sonic 2017" when first announced during Sonic's 25th Anniversary, was met with anticipation and excitement from the community due to apparently being a sequel to Sonic Generations given that Classic Sonic was also present. However, the game's hype was quickly overshadowed by Sonic Mania which was announced at the same event and over time fans began to worry about Classic Sonic being in the game due to Mania already doing the same. The character customization feature was met with some skepticism but also praise as it was taking advantage of how most people make their own Sonic characters.
The game received mixed reviews upon release. The presentation was praised, but the game was heavily criticized for bland overly-short level design, wonky physics, Classic Sonic being shoe-horned in, a very poorly executed plot, and the Avatar characters, which many have called out as inspired by the infamous Sonic OCs. Overall, the game was described by the general community as "Not terrible, but underwhelming and could not meet the expectations from the hype surrounding it."
Fans were particularly angered when they found out about the level design team, as the game had been advertised as being developed by the same team behind Sonic Colors and Sonic Generations. According to some sources, Sonic Team spent most of the four years of development working on the game engine rather than the game itself.
Jirard the Completionist was more positive than most reviewers, describing the game as "More fun to complete than to play".
An IDW Comic series which is supposed to follow up the events from the game was released that was well received.
- When the Avatar character was first teased as a silhouette, it was noted by some fans that it somewhat resembled Bubsy the Bobcat.