In Peru, Ryan helps a travel vlogger search for her friend, who has vanished while camping in a devastated river valley. In Hong Kong, Yaz and Graham team up with a troubled police officer who is tracing the whereabouts of an astronaut who disappeared following the crash of his space shuttle. In Madagascar, a research scientist witnesses the Doctor save an American naval officer who has escaped from a doomed submarine -- only to watch as the man succumbs to a terrible infection. The TARDIS crew must uncover the connection between these events, before the entire human race succumbs to an alien virus called Praxeus.