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She denied joining the plot.
Literal
She [topic-は] that plot [in-に] joined fact [object-を] denied.
否定する ('to deny, negate, repudiate') is the formal verb for refuting a claim or accusation — distinct from softer 違う ('that's wrong'). The structure ~ことを否定する uses こと-nominalization to bundle the entire embedded clause ('the fact that she joined the plot') into a single noun-like object that 否定する can take. たくらみ ('plot, scheme, plan') is the noun form of たくらむ ('to plot'), with negative connotations — distinct from neutral 計画 ('plan'). The verb 加わる ('to join, take part in') with に uses に for what one is joining.