Captain Crane is brainwashed by the enemy.
I struggled with this episode the first couple of times I saw it as it seemed like a waste of talent putting Warren Stevens in such a bland role. I know him from other things like Star Trek (By Any Other Name playing an alien), Land Of The Giants (A Place Called Earth playing a time traveller) so seeing him walk around Seaview in a suit was a turn off.
However, after yet another viewing, The Saboteur is finally loved by me as the general acting and story is well done.
The crewman named O'Brien (Derrick Lewis) is still alive and he even appears at sci-fi conventions in the USA, talking about Voyage.
Finally, the previous reviewer claims Voyage "all went to hell" in the later years, I don't agree with that at all, in fact I think the whole series is good. The second half of season one got a few boring "stinkers", but other than that, Voyage is always good or outstanding.