Summary
Highlights
While at the zoo, Johnny's truth-telling mouth convinces people that animals should be free, leading to the release of all the zoo animals into the city. The sisters realize they need to reverse the process before Johnny causes more mayhem.
After being caught by angry townspeople, Johnny uses his lying mouth to diffuse the situation by encouraging individuals to believe in themselves rather than the opinions of others. His truth-telling mouth affirms his new path, showing a change in his behavior.
Johnny's sisters use a 'Mouth of Truth 3000' device on him, intending to make him always tell the truth. However, the device malfunctions, giving Johnny two mouths: one that tells the truth and another that tells lies. This causes confusion and chaos as both mouths speak simultaneously.
Johnny and his sisters receive a letter stating they won an award for their solar-powered bicycle. However, they discover the award is from the 'Men of Science Only' organization, meaning only male scientists can accept it. To bypass this, they entered the contest under Johnny's name.
Johnny is initially unwilling to accept the award on behalf of his sisters, expressing sexist views about science. After his sisters emphasize the positive impact of their invention on energy and pollution, he agrees to help.
Due to Johnny's short attention span and disinterest in science, the sisters propose a brain swap: Susan will inhabit Johnny's body to accept the award, ensuring the presentation goes smoothly. Johnny reluctantly agrees due to a 'Johnny coupon'.
The brain swap is successful, but Susan (in Johnny's body) and Mary encounter Eugene, who misunderstands the situation and believes Susan is kidnapped. During the commotion, Susan (in Johnny's body) gets separated from Mary and ends up in trouble with Eugene.
Mary manages to convince Eugene that the person in Johnny's body is indeed Susan. They then rush to the award ceremony to claim their prize while the original Susan's body, now with Johnny's brain, is trying to escape Eugene.
At the award ceremony, Eugene reveals the body swap, exposing that a girl invented the device. The sisters and Johnny advocate for changing the discriminatory 'men-only' rule, arguing that achievement should be rewarded regardless of gender. The board reconsiders and changes their rules.
The board agrees to recognize both men and women. Afterward, Susan (in Johnny's body) and Mary decide to keep Johnny's body for a little longer so Susan can spend more time with Gil, much to the original Johnny's dismay.
Johnny's uncontrollable truth-telling causes various social disturbances, from giving unsolicited fashion advice to disrupting a wedding. The sisters pursue him to apply an 'autocorrect ray' to restore him to normal.