A Blueprint for Murder (1953)

Two orphans, Polly and Doug, live with their stepmother Lynne; Polly collapses with the same mystery symptoms that killed her father. The kids' visiting uncle, Whitney Cameron, is warned that the symptoms match strychnine poisoning, but that poisoners are seldom detected and rarely convicted. Sure enough, no case can be made against the obvious suspect; so what can Whitney do to save the next victim?