A small saver discovers the scam that threatens all subscribers to the "Caution foncière". He embarks on an incredible adventure to recover the funds paid in by his friends, and succeeds in exposing the scandal.

This story is set in the 13th century, in Britain, where Lancelot, who was raised by Vivian, the Lady of the Lake, becomes one the Knight of the Round Tables and falls secretly in love with Guinevere, the wife of King Arthur, for whom he wants to accomplish many deeds.