The core mystery revolves around the legendary missing fortune of the parish's founder, Reverend Prentice Wicks, who liquidated his wealth into a single massive diamond dubbed "Eve's Apple" and swallowed it before committing suicide. Martha Delacroix kept this secret until she confessed it during a sacrament to Monsignor Jefferson Wicks. When Wicks resolved to desecrate his grandfather's tomb to steal the diamond, Martha formed a conspiracy to eliminate him.
Martha allied with groundskeeper Samson Holt and the indebted Dr. Nat Sharp to engineer an "impossible" locked-room murder on Good Friday. Dr. Sharp administered a rapid-acting paralytic, and Wicks was stabbed inside the locked chancel closet using a devil-head dagger, framing Father Jud by using a duplicate pommel Jud had previously brought to the church. The plot unraveled when Dr. Sharp murdered Samson to hoard the gem, prompting Martha to poison Sharp's alcohol with pentobarbital. Ultimately, Benoit Blanc steps aside, allowing Martha to confess to Father Jud in the confessional booth and ingest poison, surrendering her judgment to God rather than mortal courts.