In this spin-off series, Sébastien is now 12 years of age and his beloved dog, Belle, died a few months ago leaving him lonely and upset, "inconsolable".

[3] His father sends him on vacation to the northern coastal town of Fécamp to stay with his curious old uncle, captain of a fishing vessel, named the Mary-Morgane.

[7][8] Charles Vanel, a veteran French actor who had played a desperate truck driver in Clouzot's The Wages of Fear (1953), and a restaurateur opposite Cary Grant and Grace Kelly in Alfred Hitchcock's To Catch a Thief (1955), was cast as Sébastien's eccentric but friendly fisherman uncle Louis Maréchal.

[9][10] Actress and singer Jacqueline Dano was cast as Clarisse the housekeeper, and Israeli pop star Mike Brant also appears.

[10] Capitalizing on the success of the character, the theme soundtrack to the series, composed by Daniel White and featuring the boy singing in French, was released on Philips Records, subtitled "la sirene au longs cheveux série tv".