The single player campaign starts where Red Alert 2 left off. The storyline follows the victory of the Allied campaign in RA2. Romanov's invasion was held back and the Allied forces marched into Moscow and captures Romanov. Although Romanov is no longer a threat, his subordinate Yuri, the former head of the Soviet Psychic Corps, decides to launch his own offensive. In both Allied and Soviet campaigns you work to defeat Yuri.
Even though it is just an expansion, Yuris Revenge follows the tradition of live-action cinematics and blesses the gamer with more footage of Kari Wuhrer in skimpy army outfits. Throughout the campaigns, the Allied seek help from the Soviets and vice versa in the Soviet campaign, though in the Soviet campaign they still do not consider the Allied forces as friendlies.
The gameplay mechanics and interface haven't changed since RA2, with good reason since they were perfectly fine in RA2. The new units, however, do change the way the game is played.
Soviet Siege Choppers are machine-gun mounted helicopters that can deploy an alternate fire mode where they become stationary artillery units, possibly inspired by the Siege Tanks in Starcraft. They are excellent ways to do quick hit and runs on a base if micromanaged properly. The Battle Bunkers allow Soviet infantry to garrison defensive bunkers and increase their dps. The most significant addition in Yuris Revenge for the Soviets is the Industrial Plant, which reduces the build and cost time of Soviet vehicles by 25 percent.
The Allied additions are mostly infantry. The Guardian GI is a rocket soldier version of the regular GI, having the ability to deploy a defensive stationary position. The Navy SEALS in the RA2 campaign are now trainable Allied units in Yuris Revenge's single and multiplayer. The Battle Fortress is a heavily armored APC that can fit up to 5 infantry.
Besides the new Soviet and Allied units, Yuris Revenge does something previous C&C expansions haven't, which is add a new playable side in multiplayer, Yuris faction. Yuri consists of a mix of standard infantry/tanks and infantry/tanks of psychic abilities.
While the new units and the new faction creates a whole new style of gameplay in Yuris Revenge, RA2 did a very good job of balancing the playable factions and Yuris Revenge unbalances it. The new Allied units seem to fair better against Yuri, especially the Robot Tanks since they are not mind-controllable, however both the Allied and Soviet sides have a hard time overcoming the mind control abilities of Yuris faction. Yuri has numerous different units that can MC. Although some units like the Mastermind eventually self-destructs after MCing too many units, it makes it possible for a Yuri player to MC his enemy's army and destroy all or most of his base before losing all his mind controls. Most players on Westwood Online do not even play against Yuri opponents, they will either leave the lobby, ask you to play something else, or kick you. And honestly, I can't blame them.
Despite the return of faction imbalances, this is the most content offered by a C&C expansion. Definitely check it out if you are a C&C and/or RA2 fan.