This is a very interesting Sabrina book since it basically teaches some about Irish history and mythology. Hilda is being bothered by Fergus, a leprechaun named Fergus who she has a problem with from a long time ago. He's into playing practical jokes on her and she keeps trying to catch him.
Meanwhile the school is planning a parade for St. Patrick's Day and we end up with two sets of things going on at the same time.
Fergus basically makes Sabrina, Val, Mr. Kraft and Harvey go in search of King Kevin's gold so he can take it. This involves riding a rainbow and dealing with a variety of Irish legends through time, the group having to figure out how to win over or escape that particular person for that time so they can continue until they get to the gold. That part is really interesting.
The parade portion of the story deals with shamrock cookies that allow people to see the wee folk who are trying to play tricks on the parade people. That part is really funny.
Further, there's a short section on Author's Notes which talks bout the history/mythology in the story. I really enjoyed this volume of the series.