The title story, and another, La Peur au bout du fil, were first serialised in Spirou magazine before the release in a hardcover album in 1960.

In The Traveller from the Mesozoic Era, The Count of Champignac sensationally returns from an Antarctic expedition with an intact dinosaur egg believed to be from the Jurassic period.

Back in Champignac, the Count's closest professor friends arrive (along with unpleasant atomic science genius, Sprtschk) and proceed to plan ways to hatch the egg.

With the interference of a free-roaming Marsupilami, and Fantasio acting out of sorts due to a cooling incident with an ice truck, control is soon lost, and the village of Champignac has a big dinosaur problem on its hands.

In the included story, La Peur au bout du fil, the Count continues to develop concoctions from mushrooms, but in a thoughtless moment he mistakes the toxic X4 residue from his cup of coffee, causing him to undergo big changes.