Grand Moff Tarkin is the main antagonist of Star Wars Episode IV: A New Hope. He is an Imperial commander for the Death Star. He is Darth Vader's assistant for giving commands and creating plants to destroy the Rebellion.
In the first officially launched episode of Star Wars, Grand Moff Tarkin (Peter Cushing) makes a first appearance with Darth Vader, after the Millenium Falcon had landed one one of the Death Star bays. Darth Vader feels the presence of his old master, but Tarkin reassures him that the Jedi are extinct.
He and Lord Vader held Princess Leia Organa captive on board the Death Star. They attempted to force her to reveal the location of the Death Star. After Leia told him the base was on the planet of Dantooine, Tarkin decides to destroy the princess's home planet Alderaan anyway, and then deal with her "rebel friends." After the truth is revealed that the rebel base was not at Dantooine, Tarkin planned to have Leia Organa killed.
After Princess Leia is rescued by Luke Skywalker and Han Solo, Tarkin plots with Vader the destruction of the Rebel base when in range. During the Rebel attack on the Death Star, however, Luke Skywalker destroys the Imperial space station, killing all Imperial troops, commanders, and officials onboard, including Tarkin.