The Black Guardian gave the Shadow the task of getting him the Key to Time, just as the Fourth Doctor had been sent by the White Guardian. Rather than trying to find and collect all six of the disguised segments, he located the sixth segment, and waited for over a thousand years for the Doctor to arrive with the other five.
The sixth segment was encoded in the genetic structure of the sixth royal house of Atrios, passed down through six dynasties to the sixth child, Astra of Atrios. The Shadow set Atrios to war against its neighbouring planet Zeos, taking control of the Marshal of Atrios and setting a third planet between them to prevent them from seeing the other. He hired the renegade Time Lord Drax to construct the computer Mentalis, which would control Zeos to continue to fight even when its population was completely destroyed.
He monitored the Fourth Doctor and Romana I's quest for the first five segments of the Key to Time, and waited for them to arrive to find the sixth. When they did, five years after the war had begun, he took the Doctor's TARDIS, trying to break into it with no success, and ordered the Marshal, who had captured the Doctor and Romana, to treat them well and not allow them to die yet. He ordered his Mutes to capture Princess Astra, forcing her to broadcast to Atrios that she had been captured by the Zeons. He then captured the Doctor when he tried to get to Zeos, and tried to use a control device on him to give him the other segments; when that did not work, he tortured him until the Doctor agreed to bring them to him from inside the TARDIS. The Doctor then claimed it was impossible to bring them out without the sixth piece, at which the Shadow disappeared to wait for the Doctor to make a mistake.
He tried to make Princess Astra tell him the secret of the sixth segment, using her lover Merak as leverage, without succeeding. After this, he fitted a control device on her to help him capture Merak, and then lure the Doctor and Romana to the third planet. He made her rescue the wounded Merak, and use him to get past K9 Mark II, who was guarding the TARDIS, but found it was still locked. He lured K9 away using a false distress call, and fitted another control device on it, putting it under his control. He then made Astra and Merak meet the Doctor and Romana, to try to go with him into the TARDIS, but the Doctor suggested Astra go back to Atrios to tell her people she was free. After they split up, the Shadow made her leave Merak and flee back to the Doctor, claiming she was being chased by Mutes, at last letting her gain entry to the TARDIS. The Shadow made Astria lead Romana into another trap where she was captured too, and tortured to tell him everything she knew.
He used a distress signal to lure to Doctor away, sending K9 to capture him, and took him to a cell. The Shadow then ordered Drax to get into the Doctor's cell to try to convince him that he was trying to escape from the Shadow, so they could go to the TARDIS to escape, and he could steal the Key. Instead the Doctor saw through Drax, and convinced him to work with him. The Shadow ordered K9 to bring the Doctor to him, but the Doctor pushed K9 down a slope to Drax, who removed the control device, freeing him from the Shadow, only for a Mute to capture the Doctor.
This time, the Shadow tortured Romana to convince the Doctor to bring him the first five pieces, and the Doctor reluctantly agreed. As he went, the Shadow removed the control device from Astra who was nearly ready to be transformed into the final segment. Drax shrunk the Doctor, who had just opened the door to his TARDIS, and himself; the Shadow saw this, and tried to go into the TARDIS, but was blinded by its lights, and ordered a Mute to get the Key. He took it to Astra, who transformed into the segment, but the Doctor managed to get to it first, hiding on K9 before being restored to full size by Drax, and escaping to his TARDIS.
The Doctor saw to it that a nuclear missile, fired by the Marshal's Command Capsule, struck and destroyed the Third Planet, killing the Shadow, though as he died, the Shadow reported his failure to the Black Guardian. (TV: The Armageddon Factor)
Nature and powers
Like the Black Guardian, the Shadow revelled in chaos and destruction, describing the destruction that would be caused by the Black Guardian's ultimate goal of setting the two halves of universe to destroy one another as "music in [his and the Guardian's] ears".
He employed the renegade Time Lord Drax to work for him, and communicated with the Marshal on Atrios by a secret video communications link and manipulated him, using a control device to ensure he acted exactly as required, which he later used on Princess Astra and K9.
The Shadow had an instinctive aversion to bright light, which the defence systems of the Doctor's TARDIS generated to repel him. He claimed to have waited such a long time for the Doctor that "even another thousand years" would be nothing to him.
He saw himself as the Fourth Doctor's adversary, who he believed to be inferior to him for falling into his trap, but the Black Guardian had planned that the Shadow would fail, calling him a "whimpering wraith". (TV: The Armageddon Factor)