Andy's home with the measles, and Bob is looking after him. When Donna gets home from school, Bob dashes out to be with his buddies. God pulls Bob out of a strip club and assigns him the task of putting up a billboard saying, "Smile" along the freeway. Bob protests that he can't afford it, but God assures him he'll find a way.
Bob paints "Smile" over an existing billboard ad and is caught by the police. The Devil makes Bob hit and kick the officer, and Bob finds himself on the way to jail; he tells the officer he's God's messenger, and he's taken directly to an asylum. The Devil speaks to a straitjacketed Bob and says he'll take good care of Bob's family, especially Donna, as he morphs into looking exactly like Bob. Bob's desperate to warn his family that they're in danger; he gets out into the hall and tries to phone home but is intercepted by a pair of orderlies and drugged out of his mind.
The Devil arrives at the Alman home, and Andy immediately recognizes that he isn't Bob. He tries to tell Megan, but she isn't listening. The Devil goes upstairs and quickly tries to seduce Donna, but she's having no part of it. In spite of all his pleas, she's adamant, and he collapses in defeat.
God stops by the asylum and chats with the inmates. Bob observes that it's interesting that everyone can see him. God says that the innocents of the world usually can. God's checking on Bob's progress with his assignment. God says that if anyone needs their spirits lifted it's the people in the asylum.
At the Altman house, Andy again tries to convince Megan that the man upstairs is not their dad, but Megan just makes jokes about pod people. The Devil's in the bathroom depressed that he couldn't seduce Donna. Smeck appears and tries to console him. Smeck also points out that it looks like the Devil has the measles.
The Devil loves the pampering he gets from Donna while he's recuperating from the measles. God stops by and urges him to leave this family alone, but the Devil refuses. Bob escapes from the asylum with several inmates in tow. They offer to help him fight the Devil and confide that the most powerful weapon is Tony Orlando and Dawn.
The motley group hijacks a hot air balloon decorated with a huge smiley face and heads for Detroit. They crash land into Bob's "Smile" billboard and stop traffic. As the drivers honk and wave, Bob and the other inmates dance around in delight. The group then flies over to Bob's house, and the inmates drop him off and continue on their way in the balloon.
Bob rushes in and demands that the Devil get out. The Devil isn't going anywhere; he's in love. The Devil looms into a bigger, scarier, more beastly version of himself, and it looks like Bob's in big trouble until he starts singing "Tie a yellow ribbon…" louder and louder. The Devil shrinks back to his normal and begs Bob to stop; he soon concedes defeat and leaves.
Donna comes in and orders Bob into bed. She's completely unaware that he's been away. Andy bursts in with a bucket of water, ready to fight the pod person, but realizes his dad is back and couldn't be happier.