As stated in the beginning of the film, Ruber was once a trusted knight of King Arthur and a member of the Knights of the Round Table. However, following the ten-year rule of peace and justice in Camelot, he grew obsessed with power. During a meeting, he attempts to take advantage of his rank, demanding riches which Arthur refuses to allow. Ruber then takes a stand, nominating himself for the throne of Camelot. One of Arthur's loyal Knights, Sir Lionel berates him for it, swearing that no one will serve a false king. This drives Ruber into a murderous rage, threatening to kill Arthur with a mace, but the brave Lionel defends the king at the cost of his life, just before Ruber turns to attack Arthur. However, using Excalibur, Arthur beats Ruber back who then flees Camelot in exile, vowing revenge and to one day claim Excalibur and all of Camelot to himself.
Following his ten years in exile, Ruber has gathered a swarm of barbaric followers to support his cause to take over Camelot. As the tenth year passed by, Ruber formulated a plan to steal Excalibur and use land carriages to infiltrate Camelot to exact his revenge. He attacked the lands of Lionel's widow Lady Juliana to take hold of her land wagons and forced her to tag along, threatening her daughter, the protagonist Kayley. Ruber also has his pet griffin steal Excalibur from Camelot, although the theft is bungled and the sword is dropped into the Forbidden Forest, thanks to the legendary wizard Merlin's pet silver-winged falcon Ayden. During the sacking of Lady Juliana's village, Ruber uses a potion he bought from some witches (the potion is labeled to have come from ACME) to transform his followers into an army of "iron men with hands of steel" and prepares to ride to Camelot. However, upon learning of the Excalibur's current location from the Griffin and seeing Kayley escaping into the Forbidden Forest to search for it, Ruber orders the Griffin and several of his men to follow her. During much of the film, Ruber and his posse chase Kayley (who is now assisted by Ayden, a blind hermit named Garrett, and a two-headed dragon named Devon and Cornwall), only to fall into some trouble with the forest's natural inhabitants.
Eventually, after Kayley finally finds Excalibur, Ruber and his men eventually manage to get out of the forest, and capture her. Taking the opportunity, Ruber claims Excalibur for himself, using his ACME potion to fuse it to his wrist, replacing his right arm. Delighted, Ruber continues on with his plan, gaining entrance to Camelot and attacking Arthur once more while the Griffin and the rest of Ruber's men ward off the rest of the loyal knights. Fortunately, Garrett, Ayden, Devon, and Cornwall arrive to the rescue; while Ayden, Devon, and Cornwall deal with the Griffin by burning him, Garrett takes Kayley to an underground tunnel leading to the Round Table, where Ruber is attacking Arthur before declaring himself to be the new king. During the fight, Kayley overhears Ruber talking to Arthur about the death of Lionel. Swearing that she will not serve a false king, Kayley slams a wooden beam into Ruber, launching him out of the window, and she is trapped by him when she follows. Garett attempts to strike Ruber from behind with his staff, but Ruber blocks the attack and throws Garett next to Kayley. However, he is unaware that both Garrett and Kayley are next to the stone whence Arthur once drew Excalibur before becoming king.
Ruber closes in for the kill, but when he is about to strike, the two lovers move aside and Ruber accidentally plunges Excalibur back into the stone. The power of the stone kicks in and Ruber, realizing this, attempts to pull it away to no avail, as the energy given off by the stone expunges the taint of his ACME potion from the sword. As a result, Ruber's soldiers revert to their ordinary human selves, allowing the Camelot Knights to arrest them and reclaim their kingdom. At the same time, Ruber dies as the energies between the stone and the potion conflict with each other, and disintegrates, destroyed by the very same power he sought to obtain.