Remember how Doc couldn't loan Mack and the boys his car?
He needed it to go on the trip we're going to hear about now, down to La Jolla to pick up some baby octopi. All of his friends are busy, so he has to go alone. On the way down, he picks up a hitchhiker who ends up really getting on his nerves.
When Doc was in college, he learned that sometimes people would rather hear a plausible lie than the truth.
He uses this knowledge when he goes into a restaurant and asks for something he's wanted to try for years: a beer milkshake.
Instead of telling the waitress the truth, he tells her he's sick and the doctor has prescribed beer milkshakes.