"The sinister mark" by Lee Thayer is a detective novel written in the early 20th century. The story centers on the sudden disappearance of celebrated actress Mary Blake and the troubling hints of violence left behind, drawing in her admirer Donald Van Loo Morris and private detective Peter Clancy. As a staged burglary, a blood-stained scarf, and a secretive sister named Anne come into focus, the case blends romance, identity, and danger into a tightly wound mystery. The opening of the novel follows Mary Blake’s tense supper with Donald, who confesses his love as she hints at a burden she cannot share. After slipping home alone, Mary sends Donald a frantic, intimate letter about a hidden past, imminent danger, and a decisive step she must take; she vows to return clean—or disappear forever—while warning that only her elusive sister Anne would remain. Alarmed, Donald rushes to her apartment and finds a silk scarf caught in the door smeared with blood; the rooms look looted, ashes of burned papers lie in the grate, and the kitchen window’s broken pane suggests a faked break‑in. Peter Clancy takes the case, notes signs the occupants planned to leave, discovers blood spots in the hall, misses a mysterious phone caller asking for Anne, and then tracks a taxi driver who hauled a single woman and a heavy trunk from the building to the Pennsylvania Station—likely Anne, veiled and marked by a crimson birthmark—setting the investigation in motion. (This is an automatically generated summary.)