I've recently written a screenplay about an alien invasion. A friend immediately commented that he found similarities (some general ideas, plot devices) with Half-Life 2: Ep 2, which I haven't even played - and from what he tells me, the similarities are striking.
On the other hand, I think 99% of alien invasion movies follow the same pattern - aliens invade, good guys regroup, good guys strike them with some success, aliens strike back with devastating effects, the heroes somehow save the day. There aren't that many unique plot ideas.
I'm afraid of doing anything with this screenplay, e.g. sending it to competitions, because of the "original work" clause, and the potential of getting sued. I'm 100% sure it's my original work as I've written 100% of it and the characters are original, but there's always the potential of someone saying "wait, that's actually a screen adaptation of Half-Life 2, not your original work".
To complicate things further, I understand plagiarism must include "an intention to deceive". However, there's this company that makes Snakes on a Train, Transmorphers and Alien vs Hunter which clearly intend to deceive and profit from the more famous movie's publicity, and even in that case the response seems mild.
So... what are the limits of plagiarism? Realistically, how concerned should I, a completely unknown and amateur writer, be afraid of submitting a screenplay that could be considered "derivative" to a screenwriting competition? (I'm not asking for legal advice. I know we're not lawyers here. I'm asking for practical advice out of experience). I wouldn't want my screenplay to die unread in my drawer. Thanks!