Drowning Mona is a 2000 comedy-mystery starring Danny DeVito as Wyatt Rash, a local police chief from Verplanck, New York, who investigates the mysterious death of Mona Dearly, a spiteful, loud-mouthed, cruel and around the town highly unpopular woman, who drove her son's Yugo off the cliff and drowned in the river.
Neither Phil nor Jeff seem upset when they hear about Mona's death and they are not the only ones. Ellie even wants to celebrate, because she feels the Dearlys have always treated Bobby very badly. JB Landscaping is not doing well because of Jeff's laziness and lack of right hand and because Bobby is not very competent either. Bobby denies that he still wants to fire Jeff and that he has a problem with the Dearlys.
Phil and Rona, who have an affair, meet at the Charm Motel. Phil expresses his happiness about Mona's death but denies involvement.
Bobby meets Murph and denies that he has had a hand in Mona's death. Bobby owes his brother a lot of money, but it is suggested that he is often taken advantage of by him.
Wyatt's investigation takes him to Jeff, who claims that Bobby threatened and attacked Mona. Lucinda informs Wyatt that Mona's car (i.e. Jeff's) had been tampered with in multiple ways.
Phil tells Wyatt that he was a battered husband, Mona having hit him after having accused him of having an affair. He also claims that Jeff and Mona had had an argument on the evening before the accident.
Bobby tells Wyatt that he hated Mona and that they had had an argument over Jeff's pay. Mona would not let him dissolve the partnership.
Phil and Jeff leave Mona's wake very early. Meanwhile, Wyatt breaks into the Dearly's and finds out that Mona's and Jeff's car keys have been switched.
Phil expresses his gratitude to Bobby for killing Mona. Bobby then confesses to Ellie that he rigged Jeff's car, because Jeff was destroying their business. Ellie then announces that she is pregnant. This conversation is overheard by Clarence.
Phil now tells Wyatt that he spotted Bobby near the Dearly residence on the night prior to the accident, claiming that he did not say this earlier because Wyatt and Bobby are soon to be family.
Jeff, who it turns out is also involved with Rona, finds out about Phil's affair with her. Phil later spots them making love.
Bobby tells Wyatt that Mona threatened him, not the other way around, and that he was in the Hideaway the evening before the accident, which as Valerie tells Wyatt is not true. Murph later tries to cover Bobby on this. Valerie also gives him a sharp gardening tool with the letters "JB" on it.
Phil is discovered dead in a pond at the Charm Motel. Murph tells this to Ellie, who fears that Bobby, who left their house that night, has killed again. When Rona finds out, she tries to leave town. A flashback shows Jeff not helping Phil when he fell into the water.
The police learn that Jeff is threatening suicide because of Rona's leaving. Jeff also states that Phil was not his real father and that Mona chopped off his right hand when they fought over a bottle of beer, but that in spite of all this he did not kill either of them. Wyatt manages to take the gun away from Jeff.
Wyatt then tells Bobby in private that Clarence confessed killing Phil, because he could not stand the idea of Bobby going to jail, especially with the baby on the way. A flashback shows how Phil, having spotted Bobby rig the car, tampers with it some more, which is observed by Clarence, and then, inside the house, switches the keys. (Clarence cannot have seen Phil do the latter and, presumably, thinks that Bobby too intended to kill Mona, not Jeff. Presumably, the heavy rain prevented Clarence from seeing which car was rigged, or he simply does not know which car is whose.) It must be emphasized that because of the use of flashbacks this may not be what really happened.
Wyatt promises Bobby to keep quiet about Bobby's involvement as long as he takes good care of Ellie and the baby. In the final scene, Bobby and Ellie get married and Clarence gets taken away.