- "No man can stay in the burning sun for long without having his tongue loosened."
— The Hood
Brains and Tin-Tin go on an expedition to find the lost sunken temple below the surface of 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 to claim the treasure for himself...
- Main article: Desperate Intruder/Storyline
- Main article: Desperate Intruder/Transcript
International Rescue Equipment Used
Non-International Rescue Equipment Used
- Tin-Tin wears the same diving outfit 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.)
- When Scott joins Gordon at the sunken temple ruins to help him rescue Brains, his diving gear includes what appears to be a hi-tech spearfishing gun of some sort (possibly of IR design).
- This episode features the series' smallest guest cast, with only two characters (Professor Blakely and Hassan Ali).
- 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).
- Second out of Three episodes not to feature Alan Tracy.
- On the DVDs, When Brains is hypnotized by The Hood for the first time and collapses, the film has been scanned in such a way that his glasses seemingly disappear between shots. On the US and Japanese Blu-rays, they can be seen landing on and loosely dangling from his left shoe, but the DVDs don't capture this detail. One moment the glasses are on his face, and the next they have vanished.
- During the camera swipe transition between the caravans and the first shot of Brain buried in the sand, Thunderbird 1 can be seen in the desert despite not even being launched yet. It's most likely that this episode was filmed alongside the extra scenes for the episode 'The Uninvited' , as it is the only other episode to feature Thunderbird 1 in a sandy environment.
- 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)
- Portuguese: Intruso em Desespero
- Japanese: 湖底の秘宝