Mercury International, Carlos Varela's (Carlos Thompson) company has arranged the trip of the American "Arthur Rogers Modern Jazz Quintet" for concerts in London. They are stopped and temporarily retained at the airport. When Varela goes there to learn what was the problem, he is a met by an MI5 Major (Anthony Bushell) who informs him that the group is under suspicion of transmitting sensitive US information: a trail of leaked secrets has been following the quintet on their artistic journeys abroad, to a degree that defies coincidence. However, since the search of the musician's belongings did not provide any clue, they are finally released under the recognizance of Varela, who is now held responsible of ensuring they do not commit any act of espionage while in the UK, or consequences would be bad, even to Varela. Later Varela listens the quintet's rehearsals, and begins to suspect that the information may have being transmitted in the notes of the vibraphone, an instrument played by Tania (cute English actress Dora Reisser). Varela has received an invitation for a group's recital at the London's Embassy of a foreign (and not too democratic) country, that same night. At the event, he is met by Sarah (Annika Wills) an attractive national of that country who happens to be both the niece of the foreign diplomat hosting the recital (actor Peter Arne) and an agent of the MI5 Major. Varela and Sarah go around the house and discover a tape recorder and a keyboard that could had been used to decode the messages included in the vibraphone arrangements (which the quintet always receives from the American arranger at the last minute). So it is now clear that the uncle is the head of the spies in London. Varela has his own piano retuned accordingly and, with the help of Tania (who is obviously innocent) and Sarah, decodes the first part of the message hiding in the most recent arrangement. The second part arrives with the next arrangement, but in the meantime Sarah is kidnapped by his uncle, who wants to force Varela to exchange this new arrangement for Sarah's freedom. Varela, who intended to deliver the arrangement to MI5 changes his plan when he learns that MI5 will do nothing to rescue Sarah (nor arrest the spies because they have diplomatic immunity). Varela then negotiates with the kidnappers to have the quintet play this arrangement so the uncle/spy could record it, during a garden concert, in exchange for Sarah's release. While at the concert the spies are busy recording the vibraphone solo, Sarah and Varela escape. They are immediately stopped by MI5, who believes Varela has committed espionage by allowing the spies to obtain the information. But they are released on the spot as it turns out that the vibraphone arrangement played at the recital was a version altered by Varela to render it useless to the foreign spies, and MI5 receives the right version from Varela.
