Richard Vance is an agent with Interspy.
That intelligence and enforcement organization is responsible for keeping the peace and maintaining security against all manner of dangers, both domestic and foreign. The list of opponents that it will have to deal is impressive with the most dangerous being S.P.I.D.E.R. (Society for Plunder, International Disorder, Espionage, and Racketeering).
To all outward appearances, Vance seems a fairly average fellow, tall and husky with short dark hair usually topped with a small-brimmed hat, prone to wearing a suit and tie unless undercover. Some references I have seen describe his occupation as being a detective and his manners and actions do fit that rather well. He has a nice assortment of gadgets he can and does use but for the most part, he sticks to traditional tools such as his fists and a revolver.
What makes Vance so unique and memorable is not himself per se but his shadow, hence the name of the series.
We are told that it was in Tibet that Vance learned the uncanny skill of detaching his shadow and using it to perform an amazing number of special tricks. Among those is being able to slip through tiny or extra-thin openings and disappear into other shadows and thus turn practically invisible. It can move a fair distance away from Vance's physical body so peering around corners and into adjoining rooms is common place. The major limiting factor, though, is that there must be light available to bring about this shadow form, darkness being a major impediment.
Being just a shadow, his alter-ego is for all intents and purposes invulnerable because it is not possible to hurt what does not have an actual shape. On the other hand, this incorporeal form can be dangerous as its hands have the ability to become physically solid and strike opponents as well as untie a bound Vance.