A couple more:

The villain that is Just Evil. That is, he doesn't have any justification for what he is doing, he just does bad things so The Hero has somebody to fight.

The evil government agents going after the good ET, preferably for dissection, with no concern that ETs that could manage to get here could also make WWIII look like a fun day in the park if we annoyed them sufficiently.

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I say there is an invisible elf in my backyard. How do you prove that I am wrong?

Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

The Leif Ericson Cruiser