"If Eternity Should Fail" is the 10-page Doctor Strange reprint in this issue, from STRANGE TALES #138 . The second page is shown here, with a great 2/3 page shot which is probably the ultimate of the signature other-worldly mystical dimension that Ditko defined in this series and which later artists would repeatedly imitate to capture some of the same feel. Just an amazingly organic hodge-podge of techniques defining the feature perfectly. "Inconceivable", Doc says in the script, and I think that does mean what he thinks it means.
Anyway, the story this time has Doc having found out how to reach the mysterious Eternity from a probe of the Ancient One's mind going to a journey by way of his amulet. Finding Eternity, yet another brilliant Ditko design, a creature containing the cosmos within him, he only gets some advice for his battle with Mordo and Dormammu, that he already possesses what he needs, but then returning to Earth he finds that the Ancient One has been taken by Mordo. So the issue ends with Doc facing down his foes.
As I've said before, this run of Doctor Strange is just a visual delight with every page, full of both the grant designs that define Ditkoesque and a lot of great small things (Mordo's spirit warriors rising out of the ground, the mystical portal screen that Dormammu appears in).