William Blood makes his living as a guinea-pig helping medical research into ailments such as the common cold. This is great for him as his carefree attitude means he never succumbs to the illnesses being tested. Because of this, scientists training the first group of UK astronauts recruit him so he can be the first to check out the moon, much to the annoyance of the rest of the more macho team. However, William's life may be becoming more complicated after he meets the stunning Polly, so his defence against germs could be in jeopardy.