The fact that this story mostly takes place in Honolulu certainly gives credence to the idea that this film was originally meant by 20th Century Fox to be for the Charlie Chan series. So instead of Honolulu PD's finest going undercover during an archaeological expedition its the soft spoken Japanese private investigator that does.
Peter Lorre is undercover, but that's soon blown on shipboard by G.P. Huntley playing an upper crust British twit who has a knack for turning up in the wrong place at the wrong time, but actually by accident helping Moto. Some priceless artifacts are recovered from the dig and are to be displayed in a museum in Honolulu. And there's a mysterious master crook at large known to be seeking said items for theft.
With such likely suspects as Joseph Schildkraut and Lionel Atwill in the cast you can imagine either of them as the master crook, but there are others whose behavior might make them suspicious. I will say it isn't Huntley although if he had been the brains behind all the villainy that would have been a real interesting twist.
The Moto series came to a close with relations with the Japanese getting downright unfriendly. And certainly Peter Lorre was going on to bigger and better things.