MM has always lived at arm's length from the real world. but with this episode it receives the decree absolute.
As soon as you realise that identical twins are involved you know you're in for some class hack murder writing. This episode even managed to top the "twist" (guess what--one twin was mistaken for the other!!!) with a murder that made no sense at all (a la Jonathan Creek).
****MORE SPOILING AHEAD
The crux of the episode is an old house owned by Charlie, who's in prison for a crime committed by evil twin Jack. Jack is trying to sell the house but is being thwarted by a conniving estate agent who's trying to buy the property on the cheap to do up with her evil (well, he puts satellite dishes on old houses) builder lover Gerry (yes, Gerry the builder).
The murders begin when a house-hunting couple discover the old house after being fobbed off by the conniving estate agent. Jack comes across them, but instead of offering to sell them house, garottes them one by one in their car. Why? Gos knows. And as the police pathologist asks--how did he know they'd return to the car one by one? Why didn't the husband notice his wife had been garotted when he got back into the car? Needless to say, these questions are never answered.
Meanwhile, Inspector Barnaby's wife has joined a conservation group. The conservation group has never heard of the old house, even though the builder has lodged a planning application to do it up (yes, the builder--apparently the fact that he doesn't own the place is no obstacle for him to apply for planning permission, and even the owner doesn't know). Unfortunately, despite being married to a police inspector, she has not heard of the double murder in the woods and trips off to the old house to photograph it. When Barnaby hears of this, he rushes round with the coppers. Even though the house is right by the murder scene he's previously taken no action to seal or even search it, although he knows a local kid is in the habit of visiting it, it appears to have a mysterious occupant, and the victims were garroted with wire from the piano.
At some point during all this, Jack murders the estate agent. Somebody had to do it. Charlie is found at the house and reveals that it was Jack who killed a policeman during a robbery, not him. Apparently when faced with identical twin suspect, one of whom is a greengrocer, the other a bit of a jack-the-lad, the police will arrest and convict the one dozens of people will have seen working on his stall while the robbery was committed. Charlie has absconded from prison to prevent his brother selling the house, it never having occurred to him to revoke his power of attorney by writing to his solicitor. Barnaby traps Jack by sending Charlie into the house to be garrotted.
Why (and how) Jack murdered the very people he should have offered to do a deal with is never explained. But that's evil twins for you.