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Three years after the events of Mamma Come Home, Earth is once again visited by aliens--not once, but twice. First, two aliens looking like "eighteen-foot blue space-going dinosaurs" show up on the moon, eject what seem to be three bombs covered in writing into Earth orbit, and leave. Later, a group of small, yellow aliens who claim to come from Cygnus 61 arrive, disintegrate two of the bombs (the third having been relocated to the moon for study) and land on Earth.

The 'Siggies' seem to be friendly, and go all around the Earth placing some odd maypole-like objects and singing. However, after a while, the truth comes out: the Siggies are missionaries--followers of the Great Pupa--and once they're surveyed the religions of Earth, they set about destroying major churches as a demonstration that their own god is more powerful than the gods of Earth. Not too long after, another group of Siggies (orange ones, this time) show up and engage in conflict with the first group--a religious disagreement, apparently.

At length, the protagonists send out a call for help by detonating the third 'bomb', which, they correctly surmise, was actually a beacon, and the dinosaur-like aliens were what amounts to cops. The cops come around, give the Siggies a stern warning, and thirty hours later, Earth is once more free from extra-terrestrial dangers.

Now that Earth is 'on the map', though... for how long will that remain true?

Notes

One of the missionaries, a young one named S'serrrop, comes to believe that the Great Pupa has already come to Earth in another form, but when he tries to convince his companions of this, he is fatally wounded for his trouble. I am reminded of The Fire Balloons from The Martian Chronicles, in which the missionaries discover that the natives of Mars have had their own version of Christ, in the distant past.