Captain America (1968) #305

"Walk Upon England!"

Steve is finishing up an art assignment when a strange British costume appears on his body; He takes it as a sign and wings his way to England to find out what is going on; He is attacked by Captain Britain who bests him and reveals himself to really be Modred the Mystic; Cap is chained in a dungeon with the real Captain Britain as Modred heads out to settle old scores with Merlin.