Carson Napier is half British and American. He was born in India where he learned telepathy from guru Chand Kabi. He tells his story through telepathy to Burroughs, as there might not be a chance for him to return from his adventures.
Carson Napier wanted to go to Mars to have an adventure, and he spent a great deal of time and much of his own funds building a ship for the adventure. However, forgetting to calculate the moon's gravity as a factor, Carson Napier is driven of course and finds himself headed straight for the sun. Thankfully, Venus' gravity pulls him down upon the planet instead.
Landing upon the planet, Carson soon realizes past the thick two layers of atmosphere that there is life there. Landing on a large island of vegetation he soon realizes he hasn't even landed of the surface of Venus quite yet.
Soon enough he is found by the people of Kooaad and it is here that he falls in love with Duare, the daughter of the Jong (king). However, Duare doesn't love him back. Carson Napier has many adventures upon Venus as he tries to save his beloved Duare, and to get her back home to her father, the Jong of Kooaad.
Carson Napier is a telepath, a skill he learned from Chand Kabi, and this is how he is able to tell us his adventures over such a distance. It is also said that the slightly lower gravity of Venus gives him a very minor (practically unnoticeable) boost in strength.