an already thin story boils down to surviving invaders seeking an existent anti-virus . 
(ROOT (S (NP (DT an) (ADJP (RB already) (JJ thin)) (NN story)) (VP (VBZ boils) (PRT (RP down)) (PP (IN to) (NP (NP (VBG surviving) (NNS invaders)) (VP (VBG seeking) (NP (DT an) (JJ existent) (NN anti-virus)))))) (. .)))
(ROOT (S (NP (DT ) (ADJP (RB ) (JJ )) (NN )) (VP (VBZ ) (PRT (RP )) (PP (IN ) (NP (NP (VBG ) (NNS )) (VP (VBG ) (NP (DT ) (JJ ) (NN )))))) (. )))