The Sixth Doctor is in the TARDIS mulling over the console and hitting control buttons. He reconnects the lateral balance elevation cone, disconnects the relay, and bypasses something that sounds like the dionesic diodes. The matter transporter is activated by mistake. The Doctor wonders "what foul evil" he's brought into the TARDIS this time. Tegan Jovanka appears, complete with new hairdo and her stewardess outfit. Tegan immediately recognizes where she is but not with whom. The Doctor spreads his hands out, "I've regenerated." She doesn't think it is much of an improvement. She was just sitting down to lunch when she found herself on the TARDIS. The Doctor tells her he pressed the wrong button. She quips, "The story of your life."
He needs help and she tells him she can help, "You know me, Doc, all charm, all heart..." "All mouth," adds the Doctor. He explains two Sontaran storm troopers are aboard, and Tegan asks what they are. He tells her not to the Daleks and the Cybermen, the most evil force in the universe. Tegan is sorry she asked. The Doctor asks her to disconnect the hydraulic rams and operate the ventral selenoid. Tegan wonders how turning the lights off on the Sontarans will help, "Are they afraid of the dark?" The Doctor puffs, "I can see you haven't changed a bit."
He informs her the Sontarans have a vitrox bomb on board to blow up the TARDIS. They will tap the main power cable in the TARDIS for power. The Doctor will do what he always does: improvise. Tegan rolls her eyes. She then hits a blue button but the wrong blue button. She tells the Doctor she hasn't done this for awhile. The matter transporter is activated again. "At least you won't get me arriving again," Tegan says. The Doctor finds that a comfort. The lights go off and they are jolted as the TARDIS shakes. They duck down behind the console as a small dark haired boy appears -- dressed just like the Sixth Doctor. Tegan thinks it is a sort of mini-clone. The 8 year old boy scares them with, "Boo!"
The boy tells them his name is Gareth Jenkins and he is from Earth. Knowing the dangers of the TARDIS travels, Tegan insists they get the boy back to Earth immediately. They can't risk his life. Gareth insists with the Doctor that he can help. The Doctor goes on about how he doesn't need help but then asks Gareth to help. He also asks Tegan to fetch a box. After repeating this three times, she does. Gareth stands on the box to reach the console.
The Doctor instructs the boy to set the coaxial stabilizers, charge the astral inducers, prime the mandrel condensers, and coordinate the vespian transmodifier rachet override planned stimulator. To that last one, Tegan tells the Doctor to come off it -- there is no such thing. The Doctor complains about "girls." Gareth finds it and does it. The Doctor hmm-hmm-ahmm's to Tegan, a kind of "I told you so." Tegan admits she always thought he made up the names, something the Doctor denies. The energy in the console is now fully primed.
The Sontarans blast away the door from the inner TARDIS to the control room. The three in the control room duck down from two Sontarans, who come in. The trooper has a mezon gun -- the weapon as big a rifle. These Sontarans are much taller than others the Doctor encountered as his 3rd and 4th selves. They are Group Marshall Nathan of the 10th Sontaran battle fleet and his trooper. The Doctor stands up with his two friends, after complaining about his broken door -- a perfectly good door -- and introduces them to the two Sontarans!
Gareth is known to the Sontarans and their history. The Marshall orders him killed because in the year 2001 a Sontaran fleet attacks the Earth. They are turned away due to the human leader of the forces from Earth -- Gareth Jenkins. The Doctor bluffs that it is just a name coincidence but tosses the rifle of the trooper upward and yells, "Now Gareth!" Tegan and the Doctor run to Gareth's side of the console as the boy throws a switch. They join him and all three duck behind the console. The device makes the Sontarans melt, green bile coming out of their mouths, their bodies reduced to nothing but suits and balloon like skin.
The Doctor asks Gareth how he knew to operate the TARDIS console. Gareth has seen the Doctor fly the TARDIS on telly. Tegan and the Doctor react in horror to a white haired man's face as it appears on the viewscreen scanner. The TARDIS doors open and this man, Jimmy Saville, walks in. Jimmy kisses Tegan's hand to which the Doctor seems to scoff at. Gareth passed with what the Doctor calls "flying colors" -- a reference to the outlandish jackets he and Gareth both wear. Gareth tells Jim his nan made the outfit he wears. Jim gives the Doctor a medal to put over Gareth's head to wear around his neck, "Now you are well and truly fixed." They also give him the Sontaran Mezon-gun-rifle.