An unwelcome inheritance split
Barbara leaves money to Bruce and the house to Dennis, provided Frank never enters. Dee receives nothing.
Supporting source ↗CASE FILE · S03 E03

Dennis and Dee's abrasive mother has died, and the bad news is that she stiffed them in her will, leaving all her money to her humanitarian paramour, Bruce Mathis. But at least Dennis got her house. It's a "party mansion," and that's how he intends to use it.
THE SHORT VERSION · SPOILERS AHEAD
Barbara leaves money to Bruce and the house to Dennis, provided Frank never enters. Dee receives nothing.
Supporting source ↗The men recruit friends with a badly shaped flyer. Charlie bonds with Ernesto over Dee’s diary.
Supporting source ↗Frank poses as Dee’s fiancé to recover the money. Bruce recognizes him and tests their commitment.
Supporting source ↗Bruce takes Dennis’s wedding footage as proof that Frank entered the property, threatening Dennis’s inheritance.
Supporting source ↗THE EVIDENCE
Bruce conducts the ceremony and reveals that he recognizes Frank.Source ↗
Charlie opens Dee’s childhood diary in the family house.Source ↗
Dee stands in a white dress and veil at the garden ceremony.Source ↗
Dennis records the ceremony on a handheld camera.Source ↗
Frank wears a patterned shirt while posing as Seamus.Source ↗
The Lawyer reads Barbara’s will to Frank, Dennis and Dee.Source ↗
Frank and Dee sit with Bruce’s children in her living room.Source ↗
The family sits across the desk from the Lawyer in his office.Source ↗
The gang examines a set of knives at the bar.Source ↗
Dennis hands out the party flyer among racks of athletic clothing and sporting goods.Source ↗
The large Reynolds house becomes Dennis’s proposed party venue.Source ↗
Dennis says he recruited guests at Penn.Source ↗
Dennis films the wedding and its confrontation.Source ↗
Charlie examines the illustrated diary he found in Dee’s old room.Source ↗
Rows of dumbbells stand beside Mac and Dennis in the sporting-goods store.Source ↗
Mac works at a laptop while Charlie and Dennis stand beside him.Source ↗
The Lawyer has an open document file on his desk.Source ↗
Dennis and Mac hold red disposable cups at the mansion party.Source ↗
A large teddy bear sits behind Charlie on the bed as Ernesto reads.Source ↗
Dennis’s prospective guest examines the oddly shaped paper invitation.Source ↗
Frank poses as Dee’s fiancé, Seamus.Source ↗
Frank and Dee scheme to recover Barbara’s money.Source ↗
Dee’s diary recalls her school back brace.Source ↗
Dee and Frank wear bathrobes during Bruce’s supposed relationship exercise.Source ↗
Frank’s Seamus disguise includes an attached ponytail.Source ↗
Frank sits in Dee’s apartment wearing a matching purple jacket and trousers.Source ↗
Dee wears a white bridal dress and veil.Source ↗
Dennis compares their antics to Jackass.Source ↗
Tags document researched occurrences, not every appearance. Read the research notes and attribution.