If you have the original C&C Red Alert, you definitely need to grab this.
Except for the mediocre expansion packs, that is. The Covert Operations add-on couldn't really be considered an expansion-with only 15 hear-tearing difficulty missions, a few multiplayer maps, and a new soundtrack, it was definitely not worth the purchase. Counterstrike was alright, but the new soundtracks and new missions couldn't save the game from nothing new in the player's arsenal, save for a few single-player exclusive units.
Westwood learned from its mistakes, and heck, The Aftermath is a passable expansion pack.
Story:
There isn't one. Who cares? There are a few new campaign missions, but they don't really fit into the story.
Gameplay:
If you played C&C: Red Alert, you know the drill. If not, go back and play that.
However, the new units is where the game really shines at. The Soviets get the Shock Trooper, a heavy infantry that decimates ground forces with a powerful Tesla Gun. His stronger variant, the Tesla Tank, is essentially a Tesla Coil on wheels. In addition, they get a boost of their navy with the addition of the Missile Sub.
The Allies aren't lagging behind, either. The Mechanic, a lifesaver for vehicles, is definitely an addition for any tank battalion. The Chrono Tank, essentially a Mammoth Tank without the armor, is a fearsome vehicle in the right hands. Sure, it's thinly armored, but I'd fear it if it can teleport right into your base and level your Conyard in seconds.
There are also new balance adjustments. You can no longer do mass tank rushes via mass production by building multiple war factories to reduce construction time. Bummer for rushers.
Music:
A whole new soundtrack to fit the game. Mostly techno tunes, I'm sure any C&C player will love this.
Bottom Line:
If you own Red Alert, you definitely need to get this. It gives the game a whole new boost in terms of replayability.