Star Trek: Season 2, Episode 22

2 - 22: By Any Other Name

Kelvans from the Andromeda galaxy have discovered that rising radiation levels will make their galaxy uninhabitable within ten millenia. So they have dispatched generation ships of soldiers to nearby galaxies to see which of them they might conquer. Enterprise encounters such a group when her crew answers a distress call. With the Kelvans' superior technology, they may well be able to conquer the entire Milky Way galaxy!