Garthe Knight is the son of Wilton Knight, the founder of the "Foundation for Law and Goverment", and Elizabeth Knight. Garthe's sister is Jennifer Knight. Garthe is almost indentical to Michael Knight except that Garthe has a moustache and a little beard. The reason why Garthe looks like Michael Knight is because Wilton had Michael Long's face to look like Garthe when he created the new identity "Michael Knight".
For several years Garthe was in an African prison, sentenced to three consecutive life sentences there. He meets an African Revolutionary Tsombe Kuna, a member of the "Pan-African Liberation Movement". Garthe is able to avoid his life sentences because of a top official who is taking bribes on leases on diamond mines, bribed by Garthe's mother, Elizabeth Knight. The official dies not too long afterward. Garthe and Tsombe work together for some time.
Garthe returns to the United States and works with Tsombe and his mother to steal missiles from a government installation called "Red Bluff" in Nevada. Garthe creates a semi truck which he names "Goliath" (what is similar to a Peterbilt 352 Pacemaker). Ron Wilcox, one of Garthe's employees, finds out about Garthe's plan and tries to take pictures in the compound where Goliath is kept, but he is killed by Garthe. Ron's sister, Rita, goes to the Foundation for Law and Government (FLAG) for help and later meets Michael Knight, who helps her. Elizabeth helps Garthe to get his hands on the formula for the Molecular Bonded Shell, the formula which makes Michael's car KITT invulnerable. Elizabeth drugs Devon Miles to get the partial formula and another member of the Foundation in Rio de Janeiro to get the other part of the formula; the second man dies not long afterward. Garth applies the formula to Goliath and the shell is ready. Garthe does a demonstration of Goliath for Tsombe Kuna; he rams KITT and Michael is wounded.
KITT is badly damaged, but Michael fixes KITT by rigging the car into a ramjet. Michael and KITT make it back to the Foundation and tell Devon about Goliath and what Garth is planning to do. Michael figures a way to outwit Garthe to capture him. Michael goes to the casino and challenges Garth by using Garth's ego against him; playing a casino game, with the help of KITT, Michael wins the game and Garthe is angry. He follows Michael and gets captured by Devon Miles in the Foundation's mobile unit. Michael poses as Garth to plant an explosive in Goliath. Garth escapes and captures Michael. Garth and Tsombe's soldiers proceed to Red Bluff; Goliath breaks through the wall, and they succeed in stealing the missiles. KITT manages to rescue Michael and they have a final confrontation with Goliath. KITT finds a weak spot in Goliath where the trailer hitch connects to the truck, and uses the laser that was installed by April Curtis. Garthe gets captured in the end.A year later, Garthe escapes from prison with the help of Adrianne Margeaux, a woman Michael Knight has dealt with earlier. She finances Goliath's repair, and she and Garthe work together to kidnap a Swedish scientist named Dr. Klaus Bergstrom, a brilliant physicist, to force him to use his genius for their own purposes. Garthe and Adrianne give another man plastic surgery to look like Dr. Bergstrom so that his niece, Christina, and others would not get suspicious. Garthe kidnaps Devon and April and takes them to a mansion out in the country.
Garthe's men build a special cell for Michael made out of the same stones from the African prison Garthe was in. Christina notices that her uncle, after spraining his foot earlier, is walking better and she gets suspicious. Michael goes to the hotel where Dr. Bergstrom and Christina are staying, asks him a question and senses that he is not Dr. Bergstrom. Michael manages to trace a call going out of the hotel, which leads him toward the mansion. He encounters Goliath; KITT gets hit by the truck and goes off a cliff, though KITT doesn't suffer as much damage as he did in his first encounter with Goliath. Michael manages to activate the parachute and saves them both, and then he gets captured by Garthe.Michael is put in his cell, but later manages to escape with KITT's help, and the two escape from the mansion. Michael goes to the hotel and confronts the fake Dr. Bergstrom, who is holding Christina against her will, and the impostor is apprehended by hotel security. Michael and Christina head back to the mansion to rescue Devon and April. Garthe plans to overload the systems in the mansion to kill Devon and April.
Garthe and Adrianne take Dr. Bergstrom in Goliath to meet a submarine to take the doctor out of the country. Michael arrives at the mansion and saves Devon and April by turning off the power. Devon tells Michael he called the Army to stop Goliath, but they failed to do so. Leaving Christina with Devon and April, Michael goes after Goliath to rescue Dr. Bergstrom. Michael manages to rescue Dr. Bergstrom; Garthe sees this and follows Michael. Michael and KITT go near the edge of a cliff and use the grappling hook to save themselves. Garthe is about to ram into KITT, but Adrianne, who is attracted to Michael, turns the steering wheel to prevent him from killing Michael. Goliath goes off the cliff and into the sea, killing both himself and Adrianne.