plan vs idea

plan and idea both are nouns.

Word	Noun	Adjective	Verb	Adverb
plan	Yes	No	Yes	No
idea	Yes	No	No	No

As nouns, idea is a hypernym of plan; that is, idea is a word with a broader meaning than plan:

Other hypernyms of plan include thought.

plan (noun)	idea (noun)
scale drawing of a structure	the content of cognition; the main thing you are thinking about
a series of steps to be carried out or goals to be accomplished	your intention; what you intend to do
an arrangement scheme	an approximate calculation of quantity or degree or worth
	a personal view
	(music) melodic subject of a musical composition

plan (verb)	idea (verb)
make plans for something
have the will and intention to carry out some action
make or work out a plan for; devise