- Not to be confused with the character Anthony Marston from the novel And Then There Were None.
In the novel N or M?, Anthony “Tony” Marsdon is a colleague of Deborah Beresford at the decoding department.
Anthony is described as having "sympathetic brown eyes". He is "one of the most brilliant beginners in the coding department", and it is thought that he will go far.
Anthony finds out from Deborah that Tuppence had been seen in Leahampton, and that she had "something in the sleuthing line" in the last war.
Anthony later approaches Tuppence in Leahampton, and tells her that Deborah knows that she is not in Cornwall. He also tells her that his job is "rather in the same line" as hers, and offers to help her.
Anthony later puts a proposition before Tuppence, telling her that she is to pose as a nurse, and report to Dr Binion.
It is later revealed that Anthony is a Fifth Columnist, and that the proposition he put before Tuppence was a ruse to lure her to N.