If the nasally sneer of John Lennon's voice and the metallic, rat-a-tat-tat sound of Ringo Starr's drums weren't enough to throw people ("Is that really The Beatles?!), then Paul McCartney's ginormous bass on Rain was a total head-turner.
McCartney occasionally capoed his Rickenbacker during this period, and some have speculated that he did so here. Much of his loping, octave-centered patterns sound as if they were played high on the neck. As he had done on other Beatles songs, he played a counter melody to the guitar riff - his own subtext, only this time bursting through the whole.
The interplay between McCartney and Starr at the 2:28 mark before the ride-out is as astonishing now as it was in 1966.