Operation: EndGame was the name of the plotting evil genius Dr. Ivo Robotnik's final plan to wipe out Knothole and eliminate the Knothole Freedom Fighters, specifically Sonic the Hedgehog, once and for all. The had numerous phases and was nearly carried out to completion, but due to Snively and Crocbot betraying the madman, the plan fell apart when the Ultimate Annihilator targeted only Doctor Robotnik, erasing him from existence.
When Sonic used Dr. Quack's invention - the Dream Watcher - in order to enter King Maximillian Max Acorn's psyche to contact him during his coma, Robotnik managed to track the location of Knothole when he detected the Neutron Chip in Quack's device. While Sonic and the Freedom Fighters were absent from Knothole retrieving the Sword of Acorns, Robotnik sent Snively and a group of SWATbots to apprehend Dr. Quack and King Max, with the latter being replaced by an Auto Automatron. Fearing Sonic would interfere with his plans, Robotnik appealed to former Wolf Pack member Drago Wolf to set up Sonic as a criminal for Sally Acorn's death. Dr. Quack originally refused to work for Doctor Robotnik in helping assemble the Ultimate Annihilator, but when Snively brought Dr. Quack's wife and children and threatened to roboticize them, he complied.
The first phase of Operation: EndGame was to have Princess Sally Acorn killed while framing Sonic for her murder. Drago managed to convince Hershey Cat to wear a suit that made her look like Sonic, while the suit's eyes made it so anyone Hershey saw while wearing the suit looked like Snively. During one of their missions in Robotropolis, Sally ended up hanging on to a rope high above the ground. Hershey, believing it to be Snively, cut the rope, causing Sally to fall to what appeared to be her death. With everyone witnessing what they believed was Sonic killing Sally, the Freedom Fighters targeted Sonic as a fugitive.
Robotnik used the robotic duplicate of King Max to order the Royal Secret Service to track down and apprehend Sonic. While Knothole's main defence was absent, Robotnik's army of SWATbots invaded the village, holding its citizens prisoners.
Dr. Robotnik and Sonic exchange their final blows as Robotnik is erased from existence by his own reprogrammed weapon.
Originally, Robotnik planned to fire the Ultimate Annihilator and wipe out the hostages in Knothole. He first tested the weapon on an Overlander who had been captured from Mega Central, which proved the device worked. The only thing missing was the power source, a rare ore from Downunda delivered by his treacherous Sub-Boss Crocbot, who unknown to him was planing to kill him when the ore arrived in Robotropolis. However, when Dulcy informed the Secret Service and Knuckles the Echidna of Sonic's innocence with regards to Sally's murder, the group returned and liberated Knothole before Robotnik could fire the weapon. Despite this, Robotnik planned to erase Knothole while Sonic went to Robotropolis to avenge Sally's death. Although Robotnik managed to activate his weapon, he was unaware Snively had reprogrammed the weapon to only target him. Instead of being ultimately annihilated, Knothole was trapped in a strange time zone. Bunnie Rabbot and Antoine D'Coolette placed the explosive that Crocbot originally planted in his Combot commander in the Ultimate Annihilator. After a brief fist fight with Sonic, the Ultimate Annihilator exploded, erasing Robotnik from existence.
Upon returning to Knothole, Sonic learned that Sally had in fact survived the fall, but was in a coma and in a stasis chamber. No longer a wanted fugitive as Dulcy had cleared his name, Sonic went to visit Sally and woke her from her coma with a kiss.
- It's name "EndGame" comes from the word "Edngame" meaning the final stage of a game such as chess or bridge when few pieces or cards remain, and the term "End Game" in BitTorrent vocabulary concerning its method for obtaining the final few pieces of a file.
- It's name "Operation: EndGame" is that of both a real-life plan by the U.S. Department of Homeland Security and the fictional World War II scheme by the immortal Vandal Savage in Justice League.
- The arc which Operation: EndGame was carried out was also called "End Game".
- It was later referred to by Dr. Eggman as "Project: EndGame" (sometimes spelled "Project: Endgame") to Drago Wolf.
- It was later referred to by Dr. Eggman as "Project:Endgame" to Drago Wolf.
- Sally was originally planned to be killed off permanently during Operation: EndGame, until SEGA forced Archie to alter the story-line after furious backlash from fans, mostly those of the Sonic SatAM TV series.
- The original end to Operation: EndGame with Sally's death was briefly referenced as a possible alternate reality in the future (when Chaos Knuckles tried to alter the timeline) by having the Knothole Freedom Fighters visit her grave.
- In Sonic SatAM, Dr. Robotnik had a final plan involving his ultimate weapon called the "Doomsday Project". It had similar concepts to Operation: EndGame and was possibly an inspiration for it as well.
- It is sometimes spelled "Operation End Game".