Sonic Frontiers

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The Starfall Islands act as the series's first open world, which writers compared to . The world retains traditional Sonic elements, such as springs, boost pads, and . The player explores the islands as they solve puzzles, such as orienting statues and speedrunning, to reveal parts of the map and obtain items. One of Sonic's objectives is to rescue three of his friends — Miles "Tails" Prower, Amy Rose, and Knuckles the Echidna — who are trapped in Cyber Space. Tails and Amy initially accompany Sonic in his investigation, while Knuckles was transported there earlier from the ruins above Angel Island. This is accomplished by obtaining various collectibles found in the open world. Collectibles include Kocos, which upgrade Sonic's speed and ring capacity; New Kocos, which are larger and upgrade Sonic's boost gauge; Red Seeds of Power and Blue Seeds of Defense, which upgrade Sonic's attack strength and damage reduction, respectively; Sound Memories, which allow new songs to be added to the Jukebox; and Memory Tokens, which are used to trigger cutscenes and conversations with Sonic's friends when they receive them. These either progress the story's plot (occasionally via ) or expand their relationships with Sonic by requesting the player to complete side quests. Sonic's friends project themselves as holograms in the real world while trapped, allowing them to interact with and advise Sonic. Action Chain Challenges, which also appear in the open zone, require the player to complete actions and increase their score. Getting an S-rank on all of them allows the player to utilize Sonic's Spin Dash.

The player battles robots throughout the islands, all in various forms; Sonic can dodge and parry attacks and use the Cyloop to make enemies easier to strike. Defeating enemies grants the player that allow the player to purchase additional moves and abilities in combat and the open world. Alongside regular, small enemies, the player battles large "Guardian" bosses. Defeating a Guardian rewards the player with portal gears that, when placed in a portal, allow them to enter Cyber Space.

Each of the 30 Cyber Space levels, which shift between third-person and side-scrolling perspectives, contains three optional objectives, including time attack, collecting rings, and collecting five red rings; completing each objective rewards players with a key required to collect a Chaos emerald. Big the Cat makes an appearance as a host for a fishing minigame, which is included in set areas of the maps, where Sonic can exchange purple coins found in the overworld to catch fish and other objects that net him "fishing tokens" based on catch quality. The tokens can then be redeemed for items and collectibles. Collecting all seven Chaos Emeralds allows the player to transform into Super Sonic, which is required to battle and defeat the game's major bosses, the Titans. Completing every island-mapping challenge and side story on each island and maxing out Sonic's stats (except for boost), or having the game's difficulty set to Hard or Extreme prior to battling Supreme, reveals a "secret" final boss fight with The End and a post-credits scene.

The game does not feature postgame content, as it resets the player's save file to just before obtaining the final Chaos Emerald, but defeating the End unlocks a boss rush mode called Battle Rush, a Cyber Space Challenge mode, and an Arcade Mode for Cyber Space stages. It also unlocks New Game +, which resets the player's save file but carries over a player's stats (except for when played on Extreme difficulty), essentially allowing the story to be replayed.

The "Final Horizon" update adds "Another Story", which adds Tails, Knuckles, and Amy as playable characters, each with their own skill sets. It also includes a new "true" final boss, a revamped version of the End, allowing Super Sonic to access cyber energy powers, such as Perfect Parry, but save slots from "Another Story" are separate from save slots from the main game. Completing this mode unlocks a "true ending" that expands upon the existing story.

Sonic, Tails and Amy investigate activity that has drawn the Chaos Emeralds to the islands, but their plane is sucked into a wormhole to Cyber Space. Sonic escapes to the islands in the real world, where a disembodied voice tasks him with finding the emeralds and destroying the island's robotic Titans to remove the boundary between the real and digital worlds. Believing this will save his friends – including Knuckles, who was transported there while exploring the ruins above Angel Island – Sonic releases their digital forms from cages created by Sage, who works to free Eggman from Cyber Space; destroying the cages causes Sonic's body to become increasingly corrupted. Sage cautions Sonic to leave, influencing the islands' mechanical guardians and Titans to attack him, but grows to sympathize with him while observing his interactions with his friends. Meanwhile, she and Eggman form a mutual familial bond.

Through visions of the local Koco, Sonic and his friends learn the history of the Ancients, who are revealed to be an extraterrestrial race whose planet was destroyed by "The End", an all-powerful entity. The Ancients used the Chaos Emeralds to escape and were drawn to the Master Emerald on Earth, but the End followed and started to wipe out their new civilization. The Titans were built to seal the End within Cyber Space, with the Ancients' essences remaining within the Koco, which become inert once Sonic and his friends help them fulfill their final desires in life.

After destroying three of the Titans and disabling the towers that maintain the spatial boundary, Sonic succumbs to his corruption and is trapped between dimensions. Released along with his friends and Eggman, Sonic's guide reveals itself to be the End, which attacks Earth using the last Titan, Supreme. Sonic's friends purge the corruption from him by sacrificing their physical forms, while Sage and Eggman help him collect the scattered Chaos Emeralds, allowing him to defeat Supreme. When the End flees into space, Sage uses Supreme to assist Sonic against the End's true form, sacrificing herself to destroy it. Sonic's friends are restored and leave the islands with him, now wishing to make a difference in their lives after their new experience. Meanwhile, Eggman uses the island's technology to revive Sage.