The Winslow family goes to watch a rocket launch and bring Marmaduke with them. The rocket is going to be piloted by a robot named Bruce. Marmaduke is teased by a cat and he chases it into the rocket. The door closes and the rocket takes off with Marmaduke inside.
Bruce keeps Marmaduke entertained while he performs his mission. Marmaduke chases the bone thrown by Bruce into another part of the ship and he accidentally releases a space suit and falls into it. It's a perfect fit and good thing because Bruce opens the back of the ship where Marmaduke is and Marmaduke floats outside the ship.
Marmaduke sees his bone floating in space and chases it to the moon and drags the space craft with him to the moon. Marmaduke digs a hole to bury the bone and buries the ship along with it. Bruce takes off his head and throws it up into space and Marmaduke chases it and pulls the ship out from under the moon dirt. Marmaduke retrieves Bruce's head and puts it on but Bruce is not acting right. Apparently his wires got crossed and they fly back to Earth.
Marmaduke is not the same after his trip into space and insists on playing astronaut with the kids all the time. They come into the house and Mr. and Mrs. Winslow play along and Mrs. Winslow offers the kids some cookies.