I looked this up because of the connection (singer John Lawton) to Uriah Heep. I daresay this rocks quite a bit harder than the contemporary Heep. If you've ever thought to yourself, "damn I wish early Black Sabbath had had an organ player like Deep Purple", then my friend prepare to have that lifelong dream satisfied.

Further proof of Led Zeppelin unoriginality: listen how similar the french horn (yes, French Horn in a hard rock song) is to the opening screams of "Immigrant Song". Every resource I can find on the web says this song came first.