In case you forgot: Gregor Samsa awoke...oh, wait, sorry. The Power Trio have picked up Nancy Hedford, an important Federation Commissioner who has contracted a deadly case of space AIDS that, for whatever reason, she wasn't inoculated against. They're trying to take her back to the Enterprise for treatment when the Galileo gets hijacked by an asteroid. Turns out some electrical being wants them to be playmates for Zefram Cochrane, and may also be in love with him. The being takes over the dying Commissioner's body and neo-Nancy and Zefram live happily ever after.
This is one of Trek's better science fiction premises: a totally inhuman lifeform falling in love with a human and doing anything to make him happy. But the awkward gender stuff in this episode is a bit cringe-worthy, even for TOS.The worst for me is: how the hell does this electric thing take over Hedford's body? Is Hedford still alive in there, trapped, unable to complete her mission or to advance the career she had chosen for herself?