The first thing I was planning to say tonight was that, in the same way that it pleased me to introduce our son to Randall and Hopkirk (Deceased) with him having absolutely no idea that one of the characters was a ghost, it pleased me hugely to show my family the most infamous episode of the Avengers franchise, wherein our heroes hunt a giant rat in the sewers. They may have been the only people to have seen this episode to have no idea what writer Dennis Spooner was going to throw at them.
The second thing I was planning to say tonight was that, in much the same way that Spooner bent the Avengers format farther than it had ever gone before with his masterpiece “Look – (stop me if you’ve heard this one) But There Were These Two Fellers…”, Spooner again took the format in a wild new direction with this story. Even with a familiar guest star like Jeremy Young at work here, this does not look or feel like The Avengers. There’s very little humor, and Steed doesn’t even don his bowler. This is a monster movie with three familiar characters in it.
And the third thing that I was planning to say tonight was that this is the episode where Purdey goes down in the sewers wearing the most hilariously unsuitable outfit for sewer-stalking that you’ve ever seen. Find yourself a woman who hunts giant rats in a Laura Ashley skirt, lads. She’ll never stop surprising you.
But then our son actually saw the story, or most of it anyway, and whatever I had to say stopped mattering so much. There’s a reason why everybody who saw this one as a kid remembers it. From the cold eyes of teenagerhood, this was “proof” that seventies Avengers was nowhere near as cool as sixties Avengers. From the colder eyes of adulthood, this was blah blah critical dissertation blah blah boring.
To a kid, this is the most terrifying hour of television ever made. Our son was scared out of his mind. Maybe when you’re an adult waiting for the rat, it’s just forty-five minutes of yeah, yeah, get on with it. When you don’t know what the heck the monster is – they tried to give the kids in the audience a billion clues, really, they did – then the director’s choices of reaction shots and screaming men about to get eaten are gobstompingly effective. At one point toward the end, Steed makes the decision between Purdey and a shotgun-wielding man he’s never seen before. Steed immediately sentences the man to death by throwing rat bait at him. By this point, our kid had already tried hiding in Mom’s lap, and behind the sofa, and leaving the room entirely. Knowing that guy’s fate was sealed was just about the living end for our son.
“That was NOT Godzilla-monster-scary, because that is a GOOD scary,” he told us. “That was a BAD-monster-scary. I will not watch ‘Gnaws’ again, not even for ten million dollars.” I assured him that none of the other sixteen episodes yet to come are anything even remotely as frightening as this. Marie sagely noted that even after he’s forgotten every other episode of this show, he will remember this one.
A few minutes later, safely tucked in for a good night’s sleep, a truck on the highway behind us let out a belching engine noise and our son rocketed out of bed and turned on every light in his room.
Fire Breathing Dimetrodon Time 
It’s funny you mention that. I was a little kid when I saw this for the first time too, and this episode and “Complex” tied for the scariest and most memorable to me. Years later, I could quote parts of those two almost verbatim