Dennis and Dee decide whether or not to pull the plug on their ailing grandfather; the gang learns of Pop-Pop's history as a Nazi; Mac, Charlie and Frank set off in search of Pop-Pop's spoils from the war.
Frank concocts a plan to undercut the garbage men union to get the contract to collect the city's trash.
In an effort to sever all ties with Maureen, Dennis and the gang plan to crash Maureen's wedding and wreak havoc.
When Charlie and Dee invade the world of the wealthy, they find romance within, but Mac, Dennis, and Frank try to get in on the action.
All go to the therapist's office to work through their mountain of issues, and Dennis' psychology is exposed as abnormal.
The gang rallies around Charlie's mom as she battles cancer; Mac, Charlie and Dee struggle with their faith; Frank loses his memory and his mind in a quest for buried treasure; a revelation that will shake Paddy's.
Frank cleans up so he can help out his old company with one last big merger; an unwritten rule of the bar gets the rest of the gang caught up in a case of mistaken identity.
The gang tries to define the difference between the real and the virtual worlds after slipping into an unhealthy obsession with computer games.
Mac and Dennis are enjoying a private dinner at Guigino's when they spot Charlie and Frank; Dee turns up at a table for one and watches the fight.