Hostages of Ultralum — Pierre Christin

Valerian and Laureline take a break from adventuring and go on a cruise, but Valerian feels uncomfortable among the idle and the powerful. He's not bored for long, though, because when a quartet of mercenaries board the cruise ship to kidnap the son of the Caliph of Iksaladam, they end up taking Laureline as well. With the almighty Caliph offering a massive reward for his son's return, Valerian's quest to rescue his girl is suddenly hindered by every bounty hunter in the galaxy…