The heroes of the story are Linda (Laureline) and Valentin (Valerian) who works/worked as space-time agents for Galaxity, the organization that runs future Earth. As space-time agents they have a ship capable of jumping in time and travelling faster than light.
As agents they get to do everything from ordinary expeditions to escorting diplomats, but much of their job has circled around mysterious events in the late twentieth century. Valentin is a man of action, while Linda is the ideal woman, intelligent, good-looking and very capable of taking care of herself and, occasionally, Valentin too.
Linda wasn't born in Galaxity as Valentin and thus not a space-time agent in the begining of the series. Valentin met her in the middle ages, or rather some middle ages where magic existed, while Valentin was hunting the renegade scientist Xombul (in the story "Bad Dreams"). Linda helped Valentin out of a tricky situation and since then followed him.
In the early albums Linda was more a follower to Valentin, but as the story goes on she emerges as the more capable out of the two agents. Still Valentin is the one with more knowledge and technical skill.
One funny observation is that in L'Ambassadeur des Ombres Valentin is a spit-and-polish soldier while Linda is a little more laid back. Later in Les Cercles du Pouvoir Linda is the one who is following protocols and being correct, perhaps mocking the policestate they are visiting.
Christin and Mézières has really succeded in telling a story that is both fascinating and capturing. The early adventures were humorous and the drawing style almost slapstick. But the story and the style of the drawings evolved. Mézières wonderful drawings like the one where Valerian confronts the creature that symbolizes the element water (right).
Did you know that Linda and Valentin met Princess Leia and Luke Skywalker in a sleazy bar...
...or that Linda has posed for Playboy?
See any resemblance with Marvel's Fantastic Four? Almost look like Mr. Fantastic and Invisible Girl/Woman made a guest appearance!