Alien seeds hitch a ride to earth in a space shuttle crew and begin to grow. When their numbers reach the Threshold amount they will be an unstoppable swarm. Alien seeds hitch a ride to earth in a space shuttle crew and begin to grow. When their numbers reach the Threshold amount they will be an unstoppable swarm. Alien seeds hitch a ride to earth in a space shuttle crew and begin to grow. When their numbers reach the Threshold amount they will be an unstoppable swarm. Alien seeds hitch a ride to earth in a space shuttle crew and begin to grow. When their numbers reach the Threshold amount they will be an unstoppable swarm.