Aired on: September 7, 2003 (JP), March 6, 2004 (US)
At the secret Guardian Units of the Nation military base Area 99, G.U.N. is hard at work on what they've dubbed "Project Beetle", using the information and technology they have been salvaging from E-23 Missile Wrist and Eggman's other robots to build their own means to fighting Eggman the next time he appears.
Meanwhile, G.U.N. agents Topaz and Rouge the Bat relax, until they soon notice a limo pull up to a jewelry store. Out of it emerges Nelson Thorndyke, who heads into the store to pick out an anniversary gift for Lindsey, his wife. His eyes soon fall upon the purple Chaos Emerald. After having it fitted into a ring and placed in a box, Nelson leaves, but not without drawing the attention of Rogue and Topaz as they realize he's got a Chaos Emerald with him.
Meanwhile, back at the Thorndyke Mansion, Chris soon gets a call from his father to come to Filmdom City, where Lindsey is busy rehearsing her latest movie, so he can surprise her for their anniversary. Meanwhile, Sonic and his friends are discussing the problems that introducing a new Chaos Emerald to the ones they already have could prove disastrous if the new emerald has not had time to adapt to the presence of the others first.
In Filmdom City, joined by Mr. Stewart, who followed Chris under the surmise of another Chaos Emerald being nearby and warning his superiors at G.U.N. of the situation, Nelson explains his plan to the others: He plans on disguising himself as the male lead in Lindsey's movie, then when she rehearses a critical scene, she'll believe Nelson is her co-star at first until she gets close enough to recognize him. That's when Nelson will present her with the Chaos Emerald as his anniversary gift to her, and then Chris, Chuck, and the others will reveal themselves and celebrate. Lindsey will be amazed at everything Nelson put together for their anniversary and embrace him in thanks. Afterwards, as he reveals the emerald, the blue Chaos Emerald that Sonic has on himself responds, resulting in a huge discharge of energy from the two emeralds, drawing the attention on Knuckles, and Dr. Eggman, who immediately has the Egg Fort II set course for Filmdom City. Despite what just happened, everyone shrugs it off as they prepare for Nelson's surprise.
When the scene comes up and Lindsey rehearses her role, everything seems to work as Nelson hoped, but when he approaches Lindsey to present her with the Chaos Emerald, things fall apart as he trips and falls, Lindsey removes his disguise out of suspicion, and when Sonic and Knuckles' emeralds react to the purple one, the energy released destroys the equipment on the sound stage before Tanaka is able to subdue the purple emerald again. Lindsey is furious, thinking that Nelson was trying to ruin the movie and her career, leading into a heated argument between Chris' parents that not even Chuck can break up as both Nelson and Lindsey tell him to stay out of it. Before things can escalate further, the Egg Fort II literally crashes the set, and during the confusion, Rouge is able to swipe the purple Chaos Emerald right out of Nelson's hands, only to lose it in a struggle to Eggman himself that involves her, Topaz, and Eggman, while Rouge and Topaz are thrown into Mr. Stewart. When confronted by Sonic and Knuckles, Eggman summons E-74 Weazo to deal with them while he escapes with the purple Chaos Emerald.
As Sonic and his friends face Weazo, it soon turns its attention to Lindsey, Chris, and Ella. Seeing his family in danger, Nelson throws himself in front of them to protect them, and an attempt by Tanaka to have Weazo face him fails miserably, to Sonic and Knuckles' embarrassment. Luckily, Sonic and Knuckles lure Weazo to where Knuckles shatters a large tank of water, causing it to swamp Weazo and not only extinguish its flamethrower, but also short it out for Sonic to deliver the killing blow.
With the threat over, Nelson reunites with Lindsey and Chris, thankful that they are alright, as they are with him, and after Chris and Lindsey assure Tanaka that his heroism in confronting Weazo still makes him proud to wear his uniform as the Thorndyke butler, everyone celebrates Nelson and Lindsey's anniversary. Meanwhile, Rouge tells Topaz that getting the Chaos Emeralds now is a top priority, no matter what the circumstances.
Emerald Anniversary provides examples of:
- The Bad Guy Wins: Although Sonic and Knuckles manage to destroy Weazo, and Nelson and Lindsey's anniversary is salvaged and celebrated as planned, Eggman still manages to get away with the purple Chaos Emerald (which Knuckles even points out).
- Enormous Engagement Ring: Nelson buys the biggest gem in a jewellery shop and has it made into a ring as an anniversary gift, unaware the gem is in fact a Chaos Emerald.
- Epic Fail: Tanaka's attempt to protect the Thorndykes by challenging E-74 Weazo to face him, only to be quickly swatted aside by the robot. Sonic's remark about it when Knuckles asks:Sonic: Too many kung-fu movies.
- Notzilla: Sonic and Weazo end up on the set of a movie featuring a Godzilla lookalike, with Weazo using a flamethrower on the giant monster.
- Shout-Out:
- When Eggman, Decoe, and Bocoe are chasing after Sonic, Knuckles, and Tails respectively, they end up on the sets of famous movies:
- Sonic and Eggman end up on the set of Jurassic Park. Sonic jumps on top of the t-rex's head, while Eggman accidentally crashes into it.
- Knuckles and Decoe go through the set of Star Wars. Knuckles uses what appears to be the Death Star (which is just a model) to jump higher in the air, while Decoe trips over it.
- Bocoe chases Tails on the Titanic. When Bocoe falls off the ship, Tails quickly grabs the former and holds him similarly to how Jack held Rose in the movie... only for the fox to drop the robot from the ship.
- Later, when Sonic is being chased by Weazo, they go through the set of a Godzilla movie.
- When Eggman, Decoe, and Bocoe are chasing after Sonic, Knuckles, and Tails respectively, they end up on the sets of famous movies:
