“Valerian and the City of a Thousand Planets” plays less like candy-colored swashbuckler like, say, James Cameron’s “Avatar” or James Gunn’s “Guardians of the Galaxy” and more like a science fiction rendering of Tom Sawyer. The plot has one common thread involving an alien race facing annihilation, but the scenes really follow Maj. Valerian (Dane Dehaan) and Sgt. Laureline (Cara Delevigne) and their misadventures building to the climax. While this structure provides too much allowance for meandering in the visual splendor of each scene (the visual effects work is driven, extensive), “Valerian” is, deep down, a throwback to campy, experimental films from the early 20th century, like those of French pioneer Georges Melies and films like The Thief of Bagdad (1940). Director Luc Besson’s giddy filmmaking is silly, contagious, and surprisingly fun, affirming the power of faith, trust, and commitment.