The aliens are coming! It's the cry at the heart of hundreds of sci fi movies. Dr. Michio Kaku asks, "What would we really do if a hostile alien race were intent on invading the Earth?" It seems like a hopeless scenario — any extraterrestrial aggressors who did show up would be far superior to us. But Michio has a plan to save the world. Professors Paul Davies and Max Tegmark are skeptical. They believe that faced with invasion by super advanced beings, our only sensible option would be immediate surrender. But Michio is no quitter and rolls out an audacious defense plan across the entire solar system. A visit to a Judo dojo provides inspiration for a final line of defense based on a martial arts tactic that he believes could save the entire human race.

• Michio Kaku