1 Iki Piki: Ha! We have successfully repulsed attack by the Cybe.
2 Iki Piki: They've left our ship alone and are fighting amongst themselves. Return us to our own region of space.
3 X: Very well. I shall return you whence you came, thousands of parsecs away.
4 Paris: Hey, we're still missing Serron.
4 Spanners: Shhh!!

"Whence" is a cool word.

Basically, it simply means "from where". You can just do a search and replace, replacing every instance of "whence" with "from where", and things should read correctly.

In particular, the correct usage is, "I shall return you whence you came," since this translates to, "I shall return you from where you came." You don't need an extra "to" or "from" anywhere.

"Whence" is one of a class of so-called pro-forms in English - a set of inflected pronouns which includes such words as "whither", "yonder", "thither", "thence", and "whomsoever". Many of these have dropped from common modern English usage, but they're still hanging around if you want to make use of them.