This is one of only two spy flicks that prolific genre director Antonio Margheriti made in the sixties. His other one was Lightning Bolt made the same year and that one is easily is the lesser of the two. At least there are some actual locations used here and Richard Harrison is a more appealing leading man than Anthony Eisley.
Harrison plays Bob Fleming, an all-American CIA agent impersonating a scientist, who's developed a new form of energy that would make all other types of fuel obsolete. There's also a female agent who keeps saving Fleming's butt in this film. She's Velka, a Russian no less, played by Susy Anderson. It's kind of funny the way she keeps chasing after Fleming saving his life time after time and he's sort of oblivious to the whole thing. She's after the formula too but is a gracious loser in the end.
Killers has a good, fun score by Carlo Savina with lots of jazzy na-na-na's and in one scene after Fleming asks a cab driver to take him somewhere with local charm, the belly dancing sequence we see next suddenly turns into a go-go fest with lots of hip dancers! There's a few gadgets used in the film too, like a ring that detects poison and little transmitters about the size of a button.
The original title of the film points to this being a 077 adventure but there is no mention of this in the English-dubbed print (Killers Are Challenged). Here you have a middle-of-the-road espionage flick (written by Ernesto Gastaldi) with a few more ups than downs, just enough to keep things interesting.