Brains and Tin-Tin go on an expedition to find the lost sunken temple in Lake Anasta in hopes of finding hidden treasure. However their quiet vacation is cut short when the Hood goes to dangerous lengths to stop them and claim the treasure himself.
- Main article: Desperate Intruder/Storyline
- Main article: Desperate Intruder/Transcript
- Jeff Tracy - Peter Dyneley
- Scott Tracy - Shane Rimmer
- Virgil Tracy - David Holliday
- Brains - David Graham
- Tin-Tin Kyrano - Christine Finn
- Gordon Tracy - David Graham
- John Tracy - Ray Barrett
- Kyrano - David Graham
- Grandma Tracy - Christine Finn
- The Hood - Ray Barrett
International Rescue Equipment UsedEdit
Non-International Rescue Equipment UsedEdit
- Tin-Tin wears the same diving outfit that she sported in Edge Of Impact.
- Brains' diving suit was previously worn by Mike Bromley, a diver in the Stingray episode Secret of the Giant Oyster. Bromley was also voiced by David Graham.
- This episode features the series smallest guest cast, with only two characters (Professor Blakely and Hassan Ali) other than the regular characters.
- Aboard Thunderbird 2, Tin-Tin has an Air Terrainean flight bag with her (presumably acquired during her fateful trip on the Fireflash Aircraft in Trapped in the Sky).
- Tin-Tin points out the pyramids during the flight to the Middle East (her next visit there is in Thunderbird 6).
- When Brains is hypnotized by The Hood for the first time and collapses, between shots his glasses simply disappear. They are subsequently shown to have fallen off, but the action is not seen on-screen; one moment the glasses are on his face, and the next they have vanished.
- French: Le trésor du lac
- German: Der versunkene Schatz
- Spanish: Un Intruso Siniestro; Intruso desesperado (DVD)
- Italian: Il tesoro perduto
- Dutch: Verdronken schat (VHS); De wanhopige indringer (VHS, DVD, TV 1966); De gevaarlijke indringer (TV 1992)
- Japanese: 湖底の秘宝