The owner of a stamp shop is murdered when excitedly talking on the phone to a contact about the rare Mauritius Penny that has turned up on a list. Cathy Gale applies for a job in the shop, and she and Steed witness another murder in an auction room.

Steed wants Mrs. Gale to impersonate Hilda Stern, a recently arrested assassin about to be used by the trans-national crime syndicate Intercrime. Things go swimmingly, until the real Hilda Stern breaks out of Holloway and confronts the imposter.

Soviet-British trade talks are coinciding with the first London concert of the brilliant young pianist, Stefan Veliko. A young girl accuses Veliko of assault, and is then murdered. If someone wants to disrupt the talks by incriminating the pianist, they seem to be going about it in the right way.