How does one make the jump that Plummer would have been Captain Kirk. The only assumption that can be made from this story is that William Shatner would not have been as well known when GR was (re-)casting TREK and he probably would have picked someone else (like Lloyd Bridges).
They were totally ready to sign Christopher Plummer as Captain Kirk, but Plummer had a clause that Shatner had
to be his understudy. The night before he had to go and shoot WNMHGB he caught a nasty case of crabs from the key grip's daughter and Shatner got the call. True story. Swear on a hunk of Bibles and such!!