The game‘s plot is that of the Galactic Civil War and the construction of the Death Star. It occurs during the events leading up to “A New Hope”.
In the Rebel campaign, the fledgling Rebel Alliance begin with an attack on the shipyards at Kuat introducing the player to basic space combat before infiltrating the planet of Wayland on a basic ground combat mission. The campaign explores how the X-Wing is pressed into the service of the Rebel Alliance, the liberation of Kashyyyk, the first whisperings of a brand new Imperial superweapon and ultimately the Battle of Yavin.
The Imperial campaign features Darth Vader and his search for the fledgling Rebel Alliance, interspersed with missions to help complete construction of the Death Star. It then transpires that the plans for the Death Star have been stolen so the player has to seek out the traitor who has passed the plans to the Rebels, intercept Princess Leia and then use the Death Star to crush the Alliance once and for all.
Star Wars: Empire at War at a Glance