There are several misdirections in this film. Each false road that it leads down is separated here by a block of white space, so please keep scrolling down to the end...
The first fake out:Andrew (Laurence Olivier) hasn't invited Milo (Michael Caine) over to defraud his insurance company, he intends to kill him.
The second fake out:Andrew isn't really planning to kill Milo, he wants to frighten him into utter humiliation.
The third fake out:"Inspector Doppler" is actually Milo in disguise, back to teach Andrew a lesson.
The fourth fake out:Thea isn't really dead. She's a willing participant in Milo's retribution against Andrew.
The actual ending:Andrew, in disgust over thinking that Milo has attempted to trick him a third time, shoots him. Unfortunately, Milo was telling the truth, and the police do know about the original episode between the two men (policemen Tarrant and Higgs are real). Milo dies, and the police arrive as the curtain falls.
But there's something wrong with this ending. Please read the Moviepooper's theory here.
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