Brett and Danny stumble onto the wreck of a light plane (complete with the now skeletal remains of its pilot) hidden deep in the wilds of Scotland, where it crashed a decade earlier.
It appears the pilot was a professor who had discovered a secret formula which could be worth millions - which they also discover in the wreckage.
It appears the Judge had set them up for their Scottish vacation in the hope they'd find the missing plane on their travels - and it now seems there are various desperate parties vying for the formula, including more than one young woman who all claim to be the dead Professor's only daughter.
This is a memorable and enjoyable episode with a strong cast - not least the lovely actresses who have guest roles, including a fun appearance from Valerie Leon as the soap-promoting 'Space Queen', complete with roadworthy space buggy!
We also later see Peter Sallis - many years later of course to find international fame providing the more talkative half of the Wallace and Gromit duo. We also see Roger's young daughter Deborah Moore in a cameo as a schoolgirl.
Danny's observational skills serve him well when he's taken hostage in a London taxi cab, and the pursuit by Brett is fun to watch as he desperately tries not to lose his partner.
The story is intriguing, and keep you guessing till the end as we wait and see exactly who will end up with the hotly-desired formula.