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She determined that the novel was based on a true story.
Literal
She [topic-は] that novel [subject-が] truth [target-に] is-based [nominalizer-こと] [object-を] tracked-down.
~に基づく ('to be based on, grounded in') is a formal verb expression marking foundation or grounding — 法律に基づく ('based on law'), 事実に基づく ('based on fact'). 突き止める ('track down, ascertain, get to the bottom of') comes from 突く ('thrust') + 止める ('stop'), with the image of pursuing something until you nail it down. The whole structure has a complex object: the nominalized clause [小説が真実に基づいていること] ('the fact that the novel is based on truth') serves as the object of 突き止める.