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In the unlikely event of rain, please don't expect me to come.
Literal
By-chance rain's case [topic-は] I [subject-が] come [quotative-とは] don't-think-[negative polite request-ないでください].
の場合は ('in the case of X') is a formal conditional frame using 場合 ('case, situation'). とは (quotative と + topic は) adds contrastive emphasis to the content of the thought — 'as for thinking that I come...' The double negative structure ('don't think that I come') is equivalent to 'don't expect me to come.'