#5 - Strange Tales 123 - “The Challenge of Loki!”
But in the early days of the Silver Age, a crossover into a character's comic book was a rare event, even within the same comics company (not counting books designed to be crossovers, like The Avengers).
But it was a tool Marvel used to great effect. Sometimes the "crossover" would last all of a panel or two, like the time Thor flew past Spider-Man and kept right on going. It wasn't unusual for heroes to call each other on the phone to ask for advice.
This issue of Strange Tales (cover date August 1964) is one I fondly remember for the double-crossover it includes. The comic's pages were split evenly between the Human Torch's solo adventures and the mystic adventures of the (relatively new) hero Dr. Strange.
But as much as I enjoyed the lead-in story, it's the backup feature that really made the comic. Dr. Strange is tricked by Loki, the Norse God of Mischief, into stealing Thor's hammer.
Strange realizes his mistake, which quickly translates into an all-out battle between a mortal and a god. It's a battle that pushes Strange right to the brink of defeat, and it's only his quick wits and his courage that allows him to survive.
This was the issue that made me a Dr. Strange fan (and Lee and Ditko fan) for life - he was a "normal" person who used his special education (magic, natch), to fight the forces of evil, succeeding thanks to his determination and intelligence. The story bristled with imagination and amazing adventures!
I've been reading his adventures faithfully every since.
In the world of comics today, it's not unusual for comic books characters from different companies to meet.