First of all let me say that any film with Maurice Chevalier will automatically get a look from me. Even though Hayley Mills was Walt Disney's number one star at the time, Chevalier's international status as a performer guaranteed him top billing. The younger cast members had a lot to do in keeping up with him.
Robert Gwillim who is a sea captain and father to Hayley Mills and Keith Hamshere has been lost at sea for many years now and who knows where in this wide world with 5/6 of it covered by ocean. But a swallowed bottle with a note that was found in the belly of a shark by scientist Maurice Chevalier sends the three of them in search of Gwillim's employer Wilfrid Hyde-White to finance an expedition to search. Said note leads them first to South America and then to Australia because Maurice misreads the clues. He did that a lot in this film, an occupational hazard with eccentric scientists.
This was the Magic Kingdom's second dealing with a Jules Verne story, the first being the classic 20,000 Leagues Under The Sea. Sad to say while In Search Of The Castaways has some interesting moments and in spots looks more like an Indiana Jones film, it does not make it above the juvenile level. Watching it today, I expected to see Harrison Ford ride in with bullwhip cracking to save the whole kit and kaboodle of them from villain George Sanders.
Maurice's first mistake took them 10,000 miles out of their way. It's not often one gets a second chance there.