Eustace was at the supermarket with Courage. Muriel was feeling sick, so they had to get the groceries for her. However, the milk they needed was $120. "WHAT?" screamed Eustace. Courage just stood there confused. Yesterday, it was only $7 for milk. But now, it was $120! Eustace went to the manager & screamed "WHY IS THE MILK SO EXPENSIVE?" The manager said "I needed the cows for an experiment." Then, Eustace got an idea, he said "How 'bout I take part in this experiment of yours?" The manager gleamed with evil, & said "You and the dog will do perfect for this experiment." Then, the manager exclaimed "TO THE LAB!"
Courage followed Eustace & the manager down a tunnel, and entered a mysterious labatory. "Well, what am I going to do?" asked Eustace. The manager said "You'll sit in the chair on the left, but make sure to be in your underwear!" The manager then turned to Courage & whispered "Go sit in the chair on the right." Eustace & Courage did as they were told. Eustace then asked "Will this mean I'll get cheap milk?" The next moment, strange helmets covered their heads & lightning shocked them.
Courage then woke up back in the same room. The manager & Eustace were not in sight. Courage began to scratch his back, but he mostly felt skin on his back. He then found a mirror & peered into it. However, he saw Eustace in the mirror! CRASH! Courage then put on Eustace's clothes, and ran into the tunnnel, where he found Eustace in his body! Eustace then screamed "MANAGER! WHY DID YOU DO THIS TO US?"
Courage then noticed something in the corner. It was the manager & a burnt blueprint for a brain-swapping machine! They then realized that, without the blueprint, they couldn't switch their brains back! Eustce (in Courage's body) steals the milk, and then the duo heads for home. With Muriel knowing what happened, Courage continued to liv in Eustace's body. Courage forever was a human & Eustace lived to be a dog.