在一颗名为51号星球的行星上，居住着一群通体绿色的高智慧生物，他们的文明高度发达，生活安宁幸福。不过他们也有自己的恐慌，那就是远在几百光年之外的地球人，他们生怕会遭到地球人的入侵和杀害。雷（Justin Long 音）是一个热衷星球研究的青年，他和地球上的小青年没什么两样，做实习报告、跟朋友聊天、追女朋友、找工作。这一日，一艘来自地球的宇航飞船降临51号星球，且刚好落在雷的院子里。宇航员查理·T·巴克（“巨石”强森 Dwayne Johnson 音）在当地引起一系列恐慌，自己也受到不小的惊吓。 　　由于长久以来的宣传，51号星球的居民生怕地球人会对自己造成伤害。万般无奈之下，巴克船长找到雷，希望他能帮助自己回到正被当地居民围观的飞船，启程返回地球。而另一方面，当地防卫军也针对“入侵”的地球人展开了行动。巴克船长最终能否回到故乡呢？Lem is just an average teenager working on getting the girl and furthering his career at the local planetarium - except that he's an alien. At least to U.S. astronaut Captain Charles T. Baker who lands on Lem's planet hoping for a quick flag plant and a hasty return to earth and his millions of screaming fans. But on this alien planet the media has tagged spacemen as brain-eating, zombie-creating monsters, causing Baker to run for his life and into Lem's house. Now it's up to the green native to get the clumsy astronaut back to his spaceship before military dictator General Grawl and mad scientist Professor Kipple manage to exterminate the Earthly visitor.