Plain is the opposite of Cliff. Cliff would make you want to Climb. Climb is related to Way. 
Brave is similar to Desperate. Desperate is related to Need. Need is related to Essential. 
Plain <--Antonym--> Cliff --CausesDesire--> Climb <--RelatedTo--> Way 
Brave <--SimilarTo--> Desperate <--RelatedTo--> Need <--RelatedTo--> Essential 