Voyager 2x7 "Parturition"

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Since Neelix and Kes first came aboard and Tom Paris was revealed to be the franchise's latest self-proclaimed ladies man, it was inevitable that an episode finally got Neelix and Paris hash it out over their rivalry (as far as Neelix always suspected) for the affections of Kes.

This is that episode.  There's also a cutesy alien baby they unite in protection of, but the main point is they finally confront each other and surprisingly it's a fairly forgettable moment, I think generally mistimed, maybe working better earlier or perhaps later, when both had been strongly defined by what Kes actually thinks (at this point she was already better defined by her relationship with The Doctor).  The one thing I'm grateful for is that Kes is not really a factor in this equation.

franchise * series * essential * character

Memory Alpha summary.