The Doctor and Ace respond to a distress call on Earth sometime in the 1990's. Apparently there's been some explosions near an archaeological dig where the sword Excaliber was thought to have been. Not only does this give Ace a semi new companion in Shou Yuing,who shares her love of explosives but the doctor with the Brigadier himself who was bought out of retirement specifically to assist him. After a journey onto an underwater spaceship it's discovered that the once legendary characters of King Arthurs round table were no legends but ancient space travellers. Their leaders Morgaine and Mordred,by the use of the monstrous Destroyer are intent on destroying the Earth to finish her final battle and that the doctor,who they perceive to be Merlin,the fame magician of their era,is another enemy for standing in their way.
In the meantime the knight Ancelyn develops a peculiar relationship with UNITS new Brigadier Bambera as they both ally with the Doctor,Ace,Lethbridge Stewart and Shou Yuing in defeating Morgaine,who intends to launch a nuclear weapon to defeat her enemy and revive King Arthur,who has actually turned to dust. Ace soon gains custody,but almost looses,of Excalibur. And is able to travel into Morgaines alternate dimension with the doctor to try to defeat them. While they are double crossed by Morgaine's powers again,it is the doctor who persuades Morgaine herself into stopping her deadly assault. Though it's the stubborn Brigadier's silver bullet that destroys the destroyer in the end. After,at the brigadiers home the Brigadiers wife,Ace,Bambera and Shou Yuing go in the doctors car Bessie on a girls night out leaving the Lethbridge Stewart,Ancelyn and the doctor on their own accords. Of course,the doctor will be doing the cooking.
This is actually one the strongest and most well rounded of all the seventh doctor stories. The presence of two UNIT Brigadiers,in a semi rivalry over which generation essentially will prevail is in contrast to the doctor and Ace's earnestness in their mission. The idea of King Arthur's court and Merlin being aliens from another dimention is a fascinating idea,hearkening back to earlier Doctor Who tales of fiction versus fact,even evoking Arthur C Clarke's famous law that "any sufficiently advanced science is indistinguishable from fiction". Humor is also a big part,particularly with the semi romantic/sparing comedy between Ancelyn and Brigadier Bambera. The Doctors succinct and to the point discussion with Morgaine at the end of the episode also brings up the anti nuclear war theme at the end of this story. Only after the doctors Helen Caldicott-like emotional yet level headed description of the medical consequences of nuclear war does Morgaine realize the futility of the weapon and decides on a new course. One of those Doctor Who stories were all the programs best elements fall right into place.