In 1997’s “Invasion” Earth is bombarded with asteroids that trigger retroviruses latent in the human genome. These viruses begin transforming people into aliens as part of planetary takeover bid. Seems straightforward.
The spanner in the aliens’ works is the small group of survivors who find a way of creating antibodies to the virus and save the day. Imagine that last sentence stretched out for two hours and you’ve got a pretty good idea what watching the film is like.