Secret Agent Mummy is an agent for, well, we really don't know.
Niall Rivers is a kid-next-door.
Let's discuss Rivers first. That is about a prosaic as I can imagine an introduction to be. But he is just a young boy, probably in his tween-years and nothing to really make mention of. In fact, we would not be talking about him at all if it wasn't for the small figurine. It was small enough to fit in the palm of young Niall's hand and it looked decidedly like a Egyptian human though a very strange face of a monkey or baboon or some such.
Setting the stage a bit better, we have Niall going around the neighborhood "collecting bric-a-brac for the school fete" that weekend and the owner of an old, smelly house had a ton of stuff to donage. Mom came out of that house carrying far too much and some things dropped and the figurine was one of them and when Niall picked it up, he got kinda zapped and saw weird colors in his head and he felt really odd for a moment. And that was all. Except the next morning he looked out at the house next door, empty for months, but what he saw was a pyramid! Mind you, his Mom and his younger sister, Ellie (he calls her Snitch for pretty obvious reasons, that little tattletale!) saw the house, not the pyramid. After that we start learning along with Niall that he can see things that others can't.
Which segues us back to Secret Agent Mummy (SAM). SAM, codenamed SAM-117, describes himself to Niall when they first formerly introduce themselves as "a two-thousand-year-old detective tracking down the enemies of my land". He does not give his actual name because "it is a very complicated name and I have forgotten it" [if he forgot it, how does he know it is complicated? Odd!]. He also lets Niall know that the pyramid already mentioned is really the "Pyra-Base", his portable Pyramid Base of Operations which travels wherever he does, "by most mystical means" and no one is supposed to be able to see it but, of course, Niall can.
SAM and his strange assistant, Mumbum (a robot mummy dog whose only organic part is his rear end - don't ask), come from another dimension, two over from ours, and he is on the hunt for a really bad guy who popped into our dimension and is causing trouble and .... well, if you need to know more, I suggest you read the stories.