Plot Summary gives away the whole thing!
The viewer doesn't learn that Simon Ashby is actually a psychotic until the end of the movie, or that Tony Ashby is an imposter until about halfway through.
shareOf course Simon Ashby is a psychotic; he's played by Oliver Reed. There's no guesswork there. But Tony Ashby being an imposter - that is something of a spoiler.
§« El sueño de la razón produce monstruos. »§
Eh, it gives away a bit, but we learn by about the halfway mark that Tony's a fake.
And Oliver Reed's performance makes it clear that there's something very wrong with Simon.
Reed makes the movie. Love his breakdown at the end, chittering and capering down the stairs.
I love to love my Lisa.