The series revolves around the adventures of the titular character, Johnny Test, an 11-year-old suburban boy who lives with his super-genius 13-year-old scientist twin sisters, Susan and Mary, in the Test residence in the fictional town of Porkbelly. Johnny often wittingly plays test subject for his genius twin sisters' various inventions and experiments, as well as be given gadgets and/or superpowers, and while at the same time reluctantly tackling and exploiting occasional problems and miscellaneous sticky situations, and as well as fighting evil villains and other enemies in the process; and occasionally, saving the world with their inventions.
A fourth season of the show debuted on Teletoon on September 10, 2009, and on Cartoon Network in the U.S. on November 9, 2009. As revealed and announced on August 24, 2010, the show was continued with a fifth season renewal which premiered on June 13, 2011.
On March 12, 2012, Cookie Jar Entertainment has revealed that a sixth season is in development with another 26 episodes, this renewal will bring the series total to 117 episodes. There are also discussions of a full-length animated TV movie based on the series.
The series ended on Christmas 2014. Re-runs ceased in mid-2014, but the series received a last hurrah on Christmas Day, including a burn-off of the last few episodes that had yet to air in America. Afterwards, the series no longer aired reruns on Cartoon Network, but it airs re-runs on Boomerang. As of 2017, reruns of the show have no longer been seen on Cartoon Network nor Boomerang.
- Test Twins
- Jillian, Daughter of Dark Vegan and has crush on Johnny.
- Black and White
- Speed McCool
- Bling-Bling Boy, Susan's abhorrent admirer.
- Sissy, Johhny's Archenemy.
- Dark Vegan
- Johnny Stopping Evil Force 5
- Wacko, Leader of the JSEF.
- Brain Freezer
- Zizrar Mole King
- Mittens and Albert
- Principal Joseph Harm
- Henry Teacherman