Lyric discussion by LuckyTown 

As I understand it, Donovan wrote this song for Cass Elliot of The Mamas and The Papas -- hence the "Fat Angel" title.

Elliot was a much beloved figure among musicians in the psychedelic sixties. Top rock stars, American and British, would congregate at her home in Laurel Canyon, to hang out and get high.

I believe the lyrics refer to the scene at her house. The rider on the silver bike is, I imagine, a stylish drug dealer who travels by motorcycle to bring his wares for all plus flowers for Mama Cass.

"Captain High at your service," indeed.

Perhaps that was even the dealer's tag line which he announced upon arrival and Donovan tucked away all these details in his mind for when he later wrote this song.

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