Dirk and Deke are a pair of aliens that crash landed on the planet Earth. Since their ship crashed, the pair pretended to be normal sixth graders, and attended the same school and class as Mac and Becky. Despite this, they still act strange, erasing a chalkboard with their heads and not reacting when a soda can hits them. After Mac and Becky discovered their spaceship, the twins reveal that they're actually sponge-headed aliens from another planet. They then force Becky and Mac into rebuilding their spaceship so that they can return to their home planet. Mac refuses, but they briefly transform his hand into a sponge until he relents.
Eventually, Mac and Becky rebuild the spaceship, but the twins try to force them to staying on the ship with them. Suddenly, rain begins to fall, causing Dirk and Deke's sponge heads to fill up with water, making them immobile. Mac and Becky push them back on the ship as it lifts off. Mac returns home, but soon finds that he forgot to put a bolt in on the ship. He then hears a loud crash.
Being sponge-headed aliens, Dirk and Deke can form their heads into whatever shape they desire. They can also use their heads to erase chalk off a chalkboard. They also possess a power in which they can turn others into sponges with the power to turn them back as well. They are also impervious to pain, like being hit by a soda can. Their only known weakness is water, which can soak up inside their heads and make them large and immobile.
Dirk and Deke are initially rather shy and quiet, often not talking very much to anyone. However, after they reveal their true selves to Mac and Becky, they are more cruel and bossy, forcing the two kids to work for them.
Dirk and Deke are described as being tall and skinny, much taller than the other students, making them look like human string beans. They have chalky white skin which adds to their alien appearance.