S1·E5 Death's Other Dominion

On a frozen planet, the Alphans encounter shipwrecked humans from a lost expedition. The expatriates from Earth enjoy an idyllic existence: they have been made immortal and have lived there for over 880 years. They invite the Alphans to join them in paradise, but dissidents reveal all is not at it seems, teaching Koenig that immortality comes with a price.