Insensato Coração (Irrational Heart) revolves around unreasonable attitudes and the consequences generated that threaten to change the course of many lives. This action filled, thrilling telenovela follows the fierce relationship of brothers Pedro and Leo Brandão.

Good-natured Pedro is a pilot who is engaged to be married but falls in love with Marina Drummond. They meet during the high jacking of a plane and together take control of the flight, allowing for a safe landing. In appreciation, Marina impulsively kisses Pedro, and they are instantly connected. But they are unaware that Marina is actually an old college friend of Pedro's fiancée, Luciana. When they find this out and decide to go separate ways an unpredictable series of events is unleashed.

Pedro faces a period of recovery after being tried and arrested for a crime that will not go unpunished and Marina must put her feelings on hold even after he is freed. Unlike his brother Pedro, Leo is unscrupulous and greedy. His perpetual envy for his brother causes him to make irrational choices that involve stealing, cheating, and kidnapping. To top it off, their mother, Wanda is extremely overprotective of Leo and makes foolish choices in her obsession for her son. But Pedro has underestimated just how clever and cunning his sibling can truly be.

Not only does the relentless Leo manage to edge aside his brother in the eyes of Marina, but he also succeeds in deceiving Norma, a naïve nurse who develops a genuine affection for him. He only keeps up the charade of their romance in a desperate bid to steal money from Norma's boss. But after serving a jail sentence for a crime she didn't commit, Norma will be driven by a strong desire for vengeance against Leo.

Before long, irrational actions transform two once-happy brothers into bitter rivals with Pedro demanding justice for Leo's misconduct.