'An Echo of Danger' by Dennis Spooner

While out on escort patrol, Phones hears a mysterious sounding through his headset. But when Stingray leaves the escort in order to investigate, the crew are unable to locate the source. Beginning to doubt his senses, Phones visits a psychiatrist - unaware that the doctor is none other than Surface Agent X20 in disguise; it seems that X20 is using a remote sounding device to make it appear that Phones is unfit for duty in the WASPs...

Don Mason (Captain Troy Tempest), Robert Easton (Lieutenant George Lee ‘Phones’ Sheridan / Agent X20), Lois Maxwell (Lieutenant Atlanta Shore), Ray Barrett (Commander Sam Shore / Lieutenant Fisher / Titan), David Graham (Doctor)

Directed by Alan Pattillo
Produced by Gerry Anderson

TX (UK): 22nd November 1964

Notes:
*Featuring Captain Troy Tempest, Lieutenant George Lee 'Phones' Sheridan, Commander Sam Shore, Lieutenant Atlanta Shore, Lieutenant Fisher and Marina