Young Lex Simms is half-alien. Like the rest of his family, he has inherited the remarkable Martian abilities of changing shape and traveling in spirit form through time and space. After Lex risks his life to rescue his relatives from their doomed planet, a violent storm nearly kills them. The older Martians plead with Lex to take them back to Mars where they expected to die of thirst anyway. However, the younger ones want to stay on Earth and become human. Meanwhile, arch-enemy Fi detects their presence, and - oh boy - is he mad! According to an agreement made eons ago, Earth belongs to Fi. He threatens to torture any non-Earthling he finds on his planet. The Martians must choose whether to return to Mars where death awaits, or stay on Earth and face Fi's terrible anger. Either way, their future looks grim.