The Doctor is working the controls in the TARDIS. He accidentally transports his former companion Tegan Jovanka on board. She is less than happy about this, but reluctantly agrees to help him. He reveals that two Sontarans are on board. They have a powerful vitrox bomb with which they intend to blow up the ship.
The Doctor then accidentally beams on board a human boy, Gareth Jenkins, who happens to be dressed in an outfit similar to his own. Gareth agrees to help in any way he can. He helps The Doctor set a trap for the Sontarans. The two aliens break into the console room. The leader introduces himself as Group Marshall Nathan and demands that the Doctor introduce his group. Upon hearing Gareth's name, Nathan reveals that in 2001 their invasion of Earth would be foiled by a brave military leader called Gareth Jenkins. If they kill him now, their future success is assured. However, Gareth springs their trap and kills the two villains.
When the Doctor asks how Gareth knew where all the TARDIS controls were, he says "well I've seen you fly the TARDIS on telly".
Jimmy Savile then appears on the scanner screen and then on the TARDIS set. The Doctor presents Gareth with his Jim'll Fix It medal and as an added treat, the prop "BBC meson gun" that the Sontarans used.
- The Doctor - Colin Baker
- Tegan Jovanka - Janet Fielding
- Gareth Jenkins - Himself
- Group Marshall Nathan - Clinton Greyn
- 2nd Sontaran - Tim Raynham
- Jimmy Savile - Himself
- A Fix with Sontarans was broadcast during the run of The Two Doctors. Savile introduces the skit as a "specially written episode with two Doctor Whos in it".
- Despite rumours to the contrary, the young Gareth Jenkins in this broadcast is not the same Gareth Jenkins who worked as a composer, audio engineer and actor for Big Finish in the 2000s.
- When Tegan arrives (wearing a flight attendant's uniform similar to the one she wore in the series, even though sometime between Time-Flight and Arc of Infinity she lost her job at the airline), the Doctor explains he has regenerated; since Tegan witnessed this before, she is not surprised. She is also shown to be somewhat familiar with TARDIS controls, in keeping with her earlier appearances.
- The Doctor asks Gareth to "co-ordinate the vespian transmogrifier ratchet override flange simulator". Tegan rolls her eyes and snaps that there's no such thing.
- The Doctor was aware that the Sontarans invade Earth in 2001.
- Gareth Jenkins reunited with Colin Baker for the first time since filming at a signing event in January 2013. 
- The format of Jim'll Fix It involved viewers asking requests of the host, Jimmy Savile. The show had wanted to do a Who 'fix' for "about four years", at one point almost getting the robot from The Visitation to do someone's homework, before new director Marcus Mortimer approached John Nathan-Turner. Nathan-Turner insisted any sketch be in character and suggested Eric Saward write it. (Doctor Who Magazine #104)
- Gareth Jenkins had his own Sixth Doctor costume made for him by his grandmother and asked to appear in a Doctor Who story as the Doctor.
- Gareth Jenkins' letter to Jimmy Savile read as follows:
- Dear Jim,
- I like Dr. Who very much because I have got Colin Baker's Doctor Who suit made by my Nan and a tent TARDIS. Please could I see Colin Baker and go inside the TARDIS.
- Yours sincerely
- Gareth Jenkins (Age 8).
- Group Marshal Nathan and his subordinate, unnamed on-screen but referred to as "Turner" in the script, were a sly reference to the then-current Doctor Who producer John Nathan-Turner. This happened in rehearsal: the script called him Group Marshal Stern. (Doctor Who Magazine #104)
- Janet Fielding was asked to participate in the sketch when it was discovered that Nicola Bryant was out of the country on holiday and unavailable to fill her role as the Doctor's then-current assistant, Peri Brown. She wore the flight attendant uniform she had on in Logopolis during most of Season 19, though since leaving the series her hair had grown much longer and she had bleached it blonde.
- Gareth Jenkins learned all his lines but Colin Baker hadn't had time, so he wrote out some of them on the TARDIS console as Jon Pertwee had - even joking "I did a Pertwee!". He claimed in interview that he'd improvised technobabble only for Gareth to repeat "not what I said but what he'd learned" (Doctor Who Magazine #323), but this is incorrect: Gareth does repeat Baker's lines.
- Colin Baker later told The Daily Mail that he found Savile "creepy and patronising", with "none of the professional respect that one would expect to be shared when two programmes combine for a special purpose". "I recall clearly the disappointment I felt for the young boy [Gareth] for whom I suspect the whole experience was daunting and overwhelming."
The mini-episode was originally included as a bonus feature on the 2003 DVD release of TV: The Two Doctors. However, in the wake of widespread allegations of paedophilia against Jimmy Savile, the host of Jim'll Fix It (who appears briefly in the mini-episode), as of 2014 BBC Worldwide has removed it from more recent pressings of the DVD. (Jenkins has said nothing inappropriate happened on A Fix with Sontarans)
The documentary Built for War, included on DVD releases of The Sontaran Experiment, features a discussion of this mini-episode that includes several clips. In its release as part of Doctor Who: The Collection - Season 12, the A Fix with Sontarans segment is edited to remove Savile's image from the TARDIS scanner and cut Baker's subsequent comment alluding to the host's character.