Saoirse Ronan proves she is stronger than a body-snatching alien in this exclusive clip from "The Host."

In the sci-fi romance flick, Ronan plays Melanie, a teenager whose world is torn apart when aliens come to Earth and take over, one human at a time. Melanie fights back and begins to overcome the alien parasite controlling her body when the desire to save her family and the boy she loves becomes too strong.

In this exclusive clip, The Seeker (Diane Kruger) tells Melanie that they are close to finding her still-human boyfriend, and her humanity shines through in a surprising way.

"The Host" invades theaters March 29.