Complex sentence: Doug's attempts to prove his innocence are thwarted at every turn, presumably by Lena, for example a gardener of the Harper's who had seen Doug and Lena together turns up murdered before the police can talk to him.
Split into simpler sentences: Doug's attempts to prove his innocence are thwarted at every turn, presumably by Lena. For example, a gardener of the Harper's who had seen Doug and Lena together turns up murdered before the police can talk to him.