Hi Grommitt
Once inside the Ministry of Import/Export, SAVE your game.
Look at everything and
talk (to Watson and anyone else) as much as possible.
[This part of the game can be a little tricky.]
You are supposed to eventually see Misteray drop the key (trident) into the aquarium but even if you don’t see him do that, it should still be in there, and either you or Watson must shoot at the aquarium so that the cat gets the fish - before you can get the key.
Hope that helps.
And good luck with this great old game !!
Cheers.
Mad
EDIT :MaG !! You beat me to it
And supplied a slightly different solution than the one I found in my old notes.