Alas this is one adventure on which I cannot join you

(From the screenplay for "Indiana Jones and the Temple of Doom")

INDIANA
Wu Han -- listen to me -- I'm
going to get you out of here.

In pain, Wu Han looks at Indy and struggles to speak --

WU HAN
Not this time, my friend...
(smiling bravely)
I followed you on many adventures --
but into the great Unknown Mystery,
I go first, Indy...

Thank you. When my friend first said the 'Alas this is one adventure on which I cannot join you' the closest quote I thought was the lines you quote from the screenplay of Temple of Doom but my friend didn't think it was that. But memory is a strange thing and we mangle things over time so I think perhaps it is!

It could well be from somewhere else, there's lots of quotations about death in Arts and Literature and they do get mashed up in one's mind.

Probably the most famous is Hamlet's soliloquy.

The undiscovere'd country from whose bourn
No traveller returns,