What's the most surreal plot or sub-plot that's been in a mainstream sitcom?
For me it has to be Father Ted and the Giant Reid and Hudd Hastings buying a fur coat and crown coat respectively with their ill-gotten gains for nobbling Chris the sheep in the sublime:
Chirpy Burpy Cheap Sheep
I've just watched it again and I am hurting from all of the laughing. Peerless.
PS
F**ken 'ell. One of the best aural gags I have ever seen / heard in the same episode.
Not necessary 'plots', but there were quite a few surreal cutaway bits in 'The Young Ones'.
This has to be the oddest sub-plot and it was in a 'straight' drama/mini-series 'Dynasty' (not unlike 'Dallas'). It aired on BBC/TV from 1981 - 1989. The series revolved around the Carringtons, a wealthy oil family living in Denver, Colorado. It starred amongst others, Joan Collins.
Anyway - out of the blue (literally) one of the characters was abducted by a UFO!!
Imagine an EastEnders character being abducted by a UFO! Even those writers wouldn't be so daft!
I don't think Mainstream gets any more surreal than One Foot in the Grave.