Sonic Forces is a 2017 platform game developed by Sonic Team and published by Sega. It was produced in commemoration of the 25th anniversary of the Sonic the Hedgehog franchise. The plot focuses on Sonic the Hedgehog joining a resistance movement formed by his friends, alongside its new rookie and another version of himself. Together, they must stop Doctor Eggman, who has conquered most of the world alongside Infinite, his newest lackey who has been empowered by the Phantom Ruby. It features three gameplay modes: "Classic", side-scrolling gameplay similar to the original Sega Genesis Sonic games, "Modern", 3D gameplay similar to Sonic Unleashed (2008) and Sonic Colors (2010), and a mode featuring the "Avatar", the player's custom character.

Development of Sonic Forces began after the release of Sonic Lost World in 2013. With the game's title chosen to signify its themes of power and teamwork, it was intended to have a darker tone compared to previous entries in the franchise and the character creation system was inspired by Sonic fan art the developers had come across over the years. The game was primarily developed by staff who had worked on Sonic Colors and Sonic Generations (2011), using an updated version of the Hedgehog Engine, used in Sonic Unleashed and Generations. It was announced, alongside Sonic Mania, under the working title of Project Sonic 2017 during the Sonic 25th anniversary event at the 2016 San Diego Comic-Con. The development of Sonic Forces, including the plot, was influenced by Sonic Mania.

Sonic Forces was released for Nintendo Switch, PlayStation 4, Windows and Xbox One on November 2017 to mixed reviews from critics. They praised its presentation, music, voice acting, character creation system and the Modern Sonic gameplay, but criticized the plot, overly automated level design and gameplay, technical problems, short length and the inclusion of Classic Sonic, with many critics calling it a disappointment in the wake of the positively received Mania, released earlier that year. The critical feedback would later influence the development of the Sonic franchise's next entry, Sonic Frontiers (2022).

Sonic Forces is an action-adventure platform game similar in gameplay and style to prior Sonic the Hedgehog games. The player's goal is to liberate the world from the newly established reign of Doctor Eggman. The game features three playable characters: two variants of Sonic the Hedgehog ("Classic" and "Modern") and the "Avatar" (the player's custom character). Each has unique abilities and attributes: Classic Sonic is restricted to side-scrolling gameplay reminiscent of the Sega Genesis Sonic games and can perform a spin dash on the ground and a drop dash in mid-air to gain speed. Modern Sonic's gameplay follows a similar style to Sonic Unleashed and Sonic Colors: he can double jump, perform a homing attack on enemies, springs and zip lines and a speed boost by collecting White Wisps. The Avatar is created from a base of seven animal types, which each possess a unique ability; they also wield a grappling hook and can equip weapons known as Wispons, including flamethrowers, whips and hammers. Depending on the Wispon, Avatars can use temporary abilities by collecting a corresponding Wisp type. Certain parts see players controlling both Modern Sonic and the Avatar, switching between them as necessary, similar to Sonic Heroes.

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.

Doctor Eggman attacks a city and Sonic sets out to stop him. Upon arriving, he is attacked by Infinite, a mercenary empowered by the Phantom Ruby. Infinite defeats Sonic, who is taken prisoner aboard the Death Egg. Without Sonic to stop him further, 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 movement with Tails, the Chaotix, Amy Rose, Silver the Hedgehog, Rouge the Bat and a survivor of Eggman's attack who is solely referred to as the "Rookie". Knuckles discovers Sonic's whereabouts and the Rookie steals a space shuttle, breaks into the Death Egg and rescues him. Back in the city, Tails is ambushed by Chaos, but is saved by Classic Sonic, who has now become marooned in the present due to the Phantom Ruby's power. The duo set out to spy on Eggman, learning about the gemstone and that Eggman has an endgame that will dispose of the resistance upon taking effect in three days.

Meanwhile, Sonic encounters Shadow, who reveals that the Shadow following Eggman is an illusionary doppelgänger created by Infinite, as are the other villains serving him, and that limitless numbers of them can be mass-produced to bolster Eggman's army. Tails infiltrates Eggman's computer network and finds the Phantom Ruby's weakness – the gemstone is directly powered by the Death Egg and will be rendered useless without it. While the Rookie stages a diversion, the Death Egg is destroyed by Classic Sonic. Believing they have the upper hand, the resistance attacks the Eggman Empire's capital, Metropolis. However, Eggman has a backup power source hidden in his empire's fortress and as the resistance make their final charge toward it, he has Infinite enact his endgame – creating an asteroid-like second sun over the planet that will eliminate everything in its path upon impact. Thinking fast, the Rookie uses a prototype Phantom Ruby they had recovered to dispel the sun, saving the heroes. Sonic and the Rookie then battle Infinite and defeat him, but Eggman unveils yet another backup plan, moving the original Phantom Ruby into his own personal battle mech and fighting the resistance directly. Working together, the Sonics and the Rookie defeat Eggman, leaving the doctor's whereabouts unknown. With the Phantom Ruby neutralized, all of the doppelgängers serving Eggman's army cease existing, Classic Sonic is returned to his proper time period, the Resistance disbands and Modern Sonic and his friends set out to restore the world to its former glory while the Rookie leaves to find their own way.

The DLC pack Episode Shadow serves as a prequel to the story, one month before the main plot begins. Shadow heads into the city towards enemy territory on Rouge's command, but E-123 Omega ends up causing chaos despite the fact that the mission is meant to be recon. As Shadow goes on, Omega starts getting confused, with his sensors becoming disabled, and his mind being corrupted before going silent. It is later revealed that Omega was badly damaged by Infinite, whom Shadow does not recognize. To help him remember, Infinite sends Shadow through a flashback of when Shadow had invaded Eggman's base.