Inspector Cadover investigates the murder of Sir George Simney. Inspector Appleby is mentioned several times. Innes’ series character Sir John Appleby is solidly in the CU already. Cadover reappeared in The Journeying Boy, which also contained references to Appleby, and then actually met Sir John in A Private View and “The Key.”
This crossover is one of hundreds covered in my book Crossovers Expanded: A Secret Chronology of the World Volume 3, which will be published by Meteor House! All three volumes are AUTHORIZED companions to Win Scott Eckert's Crossovers: A Secret Chronology of the World Volumes 1 and 2!