"An Englishman's Castle," the three-part BBC mini-series to be shown in its entirety tonight on Channel 26, starts with the fictional premise that Great Britain not only was invaded by the Nazis but -- worse than that -- eventually submitted to authoritarianism like a lamb.

So much for the tradition of indomitability and all that.

It is almost 40 years after the invasion. The Germans are keeping in the background. There are no swastikas and no jackbooted SS men. The Times remains on the streets, which is more than parliamentary democracy has achieved lately; Horrendous effects of the racial purity laws meant to eliminate all non-Aryans from the country are beginning to be forgotten. And also fading from the public consciousness is Black Friday, the 1947 uprising in which Winston Churchill was killed and after which a general amnesty was proclaimed. A few remaining terrorists, a sort of Oxbridge IRA, carry no weight.

The central figure is television producer Peter Ingram, a beneficiary of the 1947 amnesty and now writer and producer of the country's most popular soap opera, "An Englishman's Castle," a chronicle of family life in 1940. As the show's plot approaches the time of the German invasion, the German censors become uneasy about introducing a soldier named Rosenthal ("Wouldn't the name Smith better connote the notion of decency and courage?" Ingram is asked.) Based on a viewing of the first hour (the show starts at 8 p.m.), it is clear that a messy confrontation is coming on the issue of free speech versus "what is good for the people."

But serious as this subject may be, the material of the drama is not worthy of it, despite a performance of considerable intensity from Kenneth More as Ingram. The Lot is advanced with the devices of the very soap opera at which Ingram has made a career. For instance, he just happens to be having an affair with his star and she just happens to be Jewish. Further, the notion that the British would have given in so easily is absurd. Imagine Churchill living for seven years under Nazi rule before he is killed. Imagine Roosevelt tolerating a Nazi England and the elimination of millions of Englishmen.

These are improbabilities that this show takes for granted.