Recap pages are Spoilers Off. You Have Been Warned
It has been almost a year since Dr. Robotnik was banished to a far-off mushroom planet following his previous battle with Sonic the Hedgehog. His determination to return home matched only by the insanity he suffered in his isolation, he builds a device that uses the quill he managed to take from Sonic as a power source to broadcast a signal. The signal is received by a group of aliens that Robotnik quickly incapacitates in the hopes of using their Ring portal to return to Earth. There is someone standing between him and home, however — an echidna named Knuckles, who recognizes Sonic's quill. It becomes clear that the two have a mutual enemy and reach an agreement to travel to Earth, where their quarry resides...
Back on Earth, Sonic has taken to using his super speed to moonlight as a vigilante superhero, stopping an armored truck robbery in Seattle...albeit with a not-so-negligible amount of collateral damage. The following morning, Sonic's guardian, Tom Wachowski, takes Sonic on a fishing trip to warn him that he is being reckless with his power, along with an assurance that there will come a time when Sonic's power will be needed. All the young hedgehog needed to do was wait for that time...
Afterwards, Sonic sees Tom and his wife, Maddie, off as they travel to Hawaii to attend a wedding for Tom's antagonistic sister-in-law, Rachel. Left home alone for weekend, Sonic takes full advantage of his time alone to raise some lighthearted Cain. His reverie, however, is interrupted by the return of Robotnik, with Knuckles in tow. Knuckles quickly proves himself to be an equal match for Sonic in terms of power, with his fighting skills matched by the echidna's all-consuming desire to kill Sonic and reclaim the "ultimate power" for his tribe — a claim that piques the not-so-good Doctor's interest. Thankfully for our hero, Sonic receives some much needed help from an unexpected ally: Miles "Tails" Prower, a two-tailed fox who came to Earth to warn Sonic of Knuckles's arrival. Together, the two escape from the pursuing Echidna.
As Robotnik convinces Knuckles to help him seek out the "ultimate" power with the promise of helping him defeat Sonic, Sonic and Tails take refuge with a friend of Tom's, where Tails explains that Knuckles is a mighty warrior determined to defeat Sonic as revenge for the Echidna tribe, which had been wiped out in their battles with the Owl tribe, of which Sonic's original guardian, Longclaw, was a member. As they discuss their next course of action, the map Sonic carried with him when he arrived on Earth suddenly creates a hologram of Longclaw, who explains that she had Sonic sent to Earth to protect the "Master Emerald", a large green gemstone capable of transforming one's thoughts into reality-warping power. If Robotnik managed to get the Master Emerald first, it would spell disaster. Thankfully, the map points them in the right direction: the snowy tundra of Siberia.
After an unexpected dance-off at a disreputable Russian pub, Tails confides in Sonic that he looked up to the heroic hedgehog because he felt they were kindred spirits, Tails himself having been ostracized since his youth for his two tails. While Sonic wasn't sure about whether to trust Tails at first, the young fox's earnestness wins him over, and the two become the best of friends.
In the heart of the Siberian mountains, Sonic and Tails find an ancient temple that contains statues of the Owl tribe. The temple contains a magical "compass" that would reveal the location of the Master Emerald. Unfortunately, Robotnik and Knuckles are not far behind, a fight breaking out between Sonic and Knuckles for the compass. During the fight, Knuckles reveals that his family and tribe were wiped out during a fight with the Owl tribe — the same fight that led to Longclaw's death and a young Sonic escaping to Earth, leading Sonic to believe that he and Knuckles both lost their families that day. In the end, Tails is knocked unconscious by one of Robotnik's missiles, and Sonic is forced to prioritize saving him from an oncoming avalanche, leaving the compass for Robotnik to take as Sonic escapes through one of his Rings to Hawaii.
There is no time to rest, however, as Sonic is immediately captured by the Guardian Units of Nations — G.U.N. — a military unit formed to protect Earth from potential extraterrestrial threats. As it turns out, everyone at the wedding were undercover G.U.N. agents, including Rachel's fiancé! Sonic, Tails, and Tom are taken into G.U.N.'s custody, but are rescued by Maddie and a very scorned Rachel. At the same time, Robotnik and Knuckles use the compass to unveil the location of a hidden palace where the Master Emerald lies, the energy from the Emerald creating a bright green column of light that can be seen from Hawaii. Feeling guilty for putting Tails in harm's way, Sonic takes it upon himself to stop Robotnik and Knuckles on his own, brushing Tom and Maddie’s emotions in the process.
As Robotnik and Knuckles make their way through a lethal labyrinth into the innermost keep of the palace where the Master Emerald awaits, Sonic finally catches up. Determined to avenge his tribe, Knuckles fights Sonic with every ounce of his being. During the fight, Robotnik takes advantage of the distraction to claim the Master Emerald for himself, absorbing it into his body and disappearing as Knuckles realizes, all too late, that he had been deceived! With the Emerald spirited away, the palace begins to crumble into the sea. Knuckles ends up trapped under fallen rubble, forcing Sonic to dive into the water to save him, even though he was completely incapable of swimming. As thanks for saving him, Knuckles returns the favor and rescues Sonic from a watery grave, the two finally putting their differences aside as they are reunited with Tails and agree to take the fight to Robotnik.
Back in Green Hills, Robotnik makes his grand return, using the power of the Master Emerald to psychically dismantle G.U.N.'s armored vehicles and other nearby vehicles and structures to shape into the ultimate weapon: the Death Egg Robot. The giant war machine proves to be a greater threat than Sonic or his new friends can deal with on their own, but together, they had a chance. Realizing that Robotnik was obsessed with crushing him, Sonic agrees to be a diversion while Tails and Knuckles enter the Death Egg Robot and reclaim the Master Emerald. As Sonic desperately struggles to stay alive while drawing Robotnik's ire, Tails and Knuckles break into the Death Egg Robot, with Knuckles knocking the Master Emerald out of Robotnik's body.
His stamina reaching its limit, Sonic makes his way to the Master Emerald as the Death Egg Robot's backup power kicks in. Tom and Maddie catch up to Sonic, but between their lack of power and Sonic's strength failing him, it looks like this may be curtains for the Blue Blur and his found family. As Robotnik brings the foot of the Death Egg Robot down on Sonic to crush him once and for all, however, the Chaos Emeralds that were contained inside the now-shattered Master Emerald are absorbed into Sonic's body...
In a blinding flash of light, Sonic ascends to a state of supreme super power. His body now giving off a golden glow, Sonic completely destroys the Death Egg Robot, Robotnik vanishing beneath the crumbling machine. After using his reality-warping powers to summon the perfect chili dog, Sonic releases control of the Chaos Emeralds, sending them flying off into the cosmos. When asked why he would give up the ultimate power, Sonic concedes that he still has some growing up to do. With Tails and Knuckles at his side, however, the world would remain a safe — and cool — home for the friendly neighborhood hedgehog.
In the aftermath of the Death Egg Robot's rampage, G.U.N.'s search for Robotnik turns up something troubling: the mad scientist had been looking into a 50-year-old data file relating to a top secret military project...Project Shadow.
