Female US president Sheridan is killed from a mile by a masterly sniper who leaves no trace. The vice-president, now having succeeded for a short end of term, is contested by her brother, governor Wally Sheridan, who plays the national insecurity card, as the murder is only the first of series of terrorist strikes. Colonel Amos and his team are given a free hand to track down and catch the killer at all cost. Ross Tanner, who is found by a retired couple and lovingly nursed after a bad fall from a helicopter, comes to realize trough serious amnesia he's probably the sniper. Two teams track him down. La Mangouste's commando team kills the couple, tortures him and anyone known to have crossed his path but looses him again. General Carrington tells him his identity is an elaborate fake to intrude the conspiracy which uses the collateral victims at every terrorist strike, all tattooed with Roman numerals, like his XIII.

Ross Tanner successfully deals with his multiple identity mystery and the terrorist threats as well as a top-level conspiracy including the White House chief of staff, which La Mangouste fails to protect against him. Wally Sheridan is elected instead of president Galbraith, who thanks him and general Carrington. Yet 'XIII' senses something isn't right.