How she was to detain Mrs. Vandemeyer until the two men arrived, she did not know, but somehow or other it had to be done, and she must accomplish the task single-handed.
She so very badly wanted to release Mrs. Vandemeyer. 
mnli hypothesis: She so very badly wanted to release Mrs. Vandemeyer.  premise: How she was to detain Mrs. Vandemeyer until the two men arrived, she did not know, but somehow or other it had to be done, and she must accomplish the task single-handed.