The player earns emblems by completing missions and other tasks, many related to Chao raising. Collecting all the emblems unlocks a 3D version of the Green Hill Zone stage from the original Sonic the Hedgehog. The game has several two-player modes. Players may race on foot through new (or altered) levels, have shoot-'em-up battles in mechs, hunt for Master Emerald shards or race in go-karts. A few characters are playable in these modes, but not in the main game; Tikal and Chaos from the original Sonic Adventure are playable in the treasure-hunting game, as are Amy Rose and Metal Sonic in the foot-racing levels and mechs piloted by Chao and Big the Cat (replaced by a Dark Chao in Battle) in the shooting levels. Plot Learning of a secret weapon from the diary of his deceased grandfather, Professor Gerald Robotnik, Doctor Eggman infiltrates a high-security G.U.N. (Guardian Units of Nations) facility and revives it using a Chaos Emerald. The weapon – a black hedgehog and self-proclaimed "Ultimate Life Form" named Shadow – offers to help Eggman conquer the world, telling him to rendezvous at an abandoned space colony, ARK, with more Chaos Emeralds. Shadow goes to Central City, encountering G.U.N.'s forces after stealing an Emerald. Shadow has vowed to fulfill a promise to Gerald's granddaughter, Maria, right before she died, which he interprets as one of revenge. Shadow blasts through the military force and meets Sonic. After a brief confrontation, Shadow escapes and G.U.N. captures Sonic, whom they mistake for Shadow. Meanwhile, Knuckles encounters Rouge and Eggman, who both try to steal the Master Emerald. He stops them by shattering the Emerald and searches for the scattered shards to repair it. Rouge, spying for the government, heads to Eggman's base and the ARK. There, Shadow shows Eggman the Eclipse Cannon, another super-weapon created by Gerald, and discloses his plan: to charge the cannon with the Chaos Emeralds and use it to take over the world. Rouge appears, offering them a Chaos Emerald to gain their trust. Tails and Amy infiltrate G.U.N.'s island base and rescue Sonic, while Eggman, Shadow, and Rouge collect three Emeralds on the island. Eggman makes a global broadcast threatening to fire on the Earth in 24 hours if his demands are not met, demonstrating the cannon's power by using it to destroy half the Moon using their six Emeralds. Sonic, Tails, Amy, and Knuckles use a