The "deadly duo" of Doctor Doom and Prince Namor, the Sub-Mariner is one of Marvel's oldest alliances, going back to the earliest days of Marvel Comics when the team of Stan Lee and Jack Kirby were still crafting in broad strokes the broad outlines of what would become the Marvel Universe. They teamed up in the sixth issue of Fantastic Four which hit the stands in the early summer of 1962 a time which also saw the debuts of Spider-Man, Thor, and Ant-Man.
The Sub-Mariner himself had just re-emerged into the world several months earlier in the fourth issue of Fantastic Four when Johnny Storm, the Human Torch found his in Bowery suffering amnesia. Quickly Namor gathered his senses and even some reinforcements and became an adversary for the Fab 4.
Doctor Doom himself debuted in the very next issue, issue five and sent the Fantastic Four, or at least three of them on a time-traveling trip to encounter pirates and whatnot. Doom was seemingly defeated but as we all know now, those never last long.
As it turned out, it lasted just one issue and in the sixth issue of the Fantastic Four he's back and this time looking to complement his own power with that of the Sub-Mariner. Doom goes into the sea and confronts Namor in his own lair among the ruins of Atlantis and uses the grief Namor feels to get him to assist in his own scheme to plant a "grabber" in the Baxter Building. The grabber will enable Doom to get control of the entire building and move it. The Sub-Mariner goes along with the plan with the notion that he himself will rescue the Invisible Girl, a woman he has strong feelings for. But Doom of course betrays Namor and makes off with the entire Baxter Building, using his own rocket plane to take it into space. The FF working with Subby eventually concoct a scheme to use water pressure to eject Namor so he can skip along convenient meteors to access Doom's ship where he confronts the Latverian monarch and defeats him by using his ability to channel electrical current like an eel. Namor then sends Doc Doom sailing off into the depths of space. At this point the Baxter Building is returned to NYC, seemingly having suffered relatively little damage.
It's really one of the more outlandish yarns Stan and Jack ever spun about the FF and Namor's powers are off the charts in this one. Doctor Doom though is well realized and plays his nefarious part to the hilt. The "Diabolical Duo" as the title of the story labels them have now established a working relationship, albeit a brief one. That relationship will form the basis of sorts for a longer partnership when they eventually seek one another out in the pages of Giant-Size Super-Villain Team-Up.
More as Doc Doom has more schemes.