how do i re-download an episode?

so i purchased the two seasons of Falling Skies but one of the episodes on the first season: Sanctuary, Pt. 2 didnt download correctly, i dont believe. For one, i cant get it to play (it stopped in the middle and when i press play, it goes all slow-motion and audio isnt existant). secondly, it wont download onto my ipod (its an ipod classic 80g) and when it reaches that episode, it gets stuck on its number (like "downloading 4/18", for example) and it wont completely sync, and anything else that i tryed to download to it at the same time dont appear on my ipod after. i think if i can re-download it, it might fix it, but im NOT going to pay for it, that ********, i already paid for every episode. PLZ HELP