Shy high school junior Riley Kowalski is spending her winter break on a research trip to Antarctica, sponsored by one of the world's biggest tech companies. She joins five student volunteers, a company-approved chaperone, and an impartial scientist to prove that environmental plastic pollution has reached all the way to Antarctica, but what they find is something much worse... something that looks human. Riley has anxiety - ostracised by the kids at school because of panic attacks - so when she starts to feel like something's wrong with their expedition leader, Greta, she writes it off. But when Greta snaps and tries to kill Riley, she can't chalk it up to an overactive imagination anymore. Worse, after watching Greta disintegrate, only to find another student with the same affliction, she realises they haven't been infected, they've been infiltrated - by something that can change its shape. And if the group isn't careful, that something could quickly replace any of them.