It's bracing that it's been practically a decade since the Avengers were hot, hot, hot under the guiding hands of Kurt Busiek and George Perez. That's the last time I really liked the team all that much, and so did lots of folks it seems. So many that Marvel tried to get Busiek to work his charm on revitalizing the Defenders with the help of Erik Larsen. That revival tanked.
But they did hang around long enough to generate the reprint above. This book reprints two of my all-time favorite comic books, both written by my favorite comic book writer Roy "The Boy" Thomas.
One thing that defined Marvel back in the old days was the organic way they told stories. It was partly inspiration but mostly lack of planning. At its core though was a genuine care taken for the readership. If a series abruptly ended, then you could be relatively certain the storyline would pop up somewhere in the Marvel Universe so that your reading experience could be culminated. These necessary stories often proved to generate some of the line's most vivid reading experiences as it often changed up a character's status quo quite effectively.
The saga of the Lovecraftian menace of the Undying Ones is arguably the best example of this that I can think of. Dr.Strange began his journey in Strange Tales and when Marvel was able to expand its line in the 1968 he was given his own title continuing the numbering of the venerable Strange Tales series. But alas his book beautifully drawn by Gene Colan didn't sale well enough and cancellation came with issue #183 despite some pretty strong attempts to save the series, such as giving him a new swanky superhero-y suit. (This comic isn't reprinted in theDay of the Defenders, but the next two are.)
The story continues in Sub-Mariner #22 where Dr.Strange shows up to work alongside the quixotic Namor to again beat back the threat of the Undying Ones.
That Subby story ended with the threat of the Undying Ones still looming. Then came Hulk #126 and the saga of the Undying Ones continued. We meet among others the girl who would become Valkyrie and a captured Doc Strange in a story by Roy Thomas and Herb Trimpe that is a trip and half. The wacky Night-Crawler is one of Herb's strangest concoctions.
For a more detailed explanation see this link.
The upshot of all this was that Roy Thomas saw these radically different heroes seem to have a chemistry. Later still Subby, Hulk, and the Silver Surfer (then without his own book) team up in Subby issue #34 and #35.
These "Titans Three" stories drawn by Sal Buscema and again scripted by Roy are suggested to be the genesis of the Defenders and that's true to a point, but clearly the real spark were the Undying Ones stories as is proven by the fact the Non-Team take on the Undying threat and eventually it results in their first original member, the aforementioned Valkyrie.
Like I said, Marvel had a real haphazard and organic way of telling stories back in the day. This was a result of the small staff I'm sure, and in the case of these comics the fact Roy wrote all three helped immensely. But nonetheless the blend of clever and random resulting from not planning so carefully as they do these days made for real glimmers of greatness. The Defenders are proof of that.