Lloyd Christmas (Jim Carrey) and Harry Dunne (Jeff Daniels), two unintelligent men, are best friends and roommates living in Providence, Rhode Island who struggle at every aspect of life. Lloyd, a limousine driver, falls in love with Mary Swanson (Lauren Holly), a woman he is driving to the airport. She intentionally leaves a briefcase in the terminal; Lloyd, unaware that it contains ransom money for her kidnapped husband, Bobby, retrieves it and tries to return it to her, but her Aspen-bound plane has already departed, leading to Lloyd running through and falling out of the jetway.
Fired from his job, Lloyd returns to his apartment and learns that Harry has also been fired as a dog groomer after delivering dogs late to a show and accidentally getting them dirty. Bobby's kidnappers, Joe "Mental" Mentalino (Mike Starr) and J.P. Shay (Karen Duffy), follow Lloyd home from the airport in pursuit of the briefcase. Mistaking the crooks for debt collectors, the duo flee the apartment and return later to find that Mental and Shay have decapitated Harry's parakeet. Upset about their situation, Lloyd suggests they head to Aspen to find Mary and return the briefcase, hoping she can "plug them into the social pipeline." At first Harry opposes the idea, but eventually agrees and the duo leave the next day. Mental and Shay learn about their plans and follow them.
Mental and Shay catch up to the duo at a motel that night. Posing as a hitchhiker, Mental is picked up by Harry and Lloyd the next day only to be driven crazy by their childish antics, while Shay secretly follows them. During a lunch stop, the duo prank Mental with chili peppers in his burger and then accidentally kill him with rat poison pills (which he planned to use on them) after mistaking it for his medication. Nearing Colorado, Lloyd takes a wrong turn and ends up driving all night through Nebraska. Upon waking up and realizing Lloyd's mishap, Harry gives up on the journey and decides to walk home, but Lloyd later persuades him to continue after trading the van for a minibike.
The two arrive in Aspen but are unable to locate Mary. After a short scuffle over some gloves that night, the briefcase breaks open and they discover the money, and "borrow" it for a hotel suite, clothes and a Lamborghini Diablo. They learn that Mary and her family are hosting a gala and prepare to attend. At the gala, Harry, attempting to lure Mary over to Lloyd, reluctantly agrees to go skiing with her the next day and lies to Lloyd that he got him a date. The next day, Lloyd finds out Harry lied to him after waiting all day for Mary at the hotel bar.
In retaliation, Lloyd pranks Harry with coffee spiked with laxatives, leading to him to defecate in a broken toilet at Mary's house. Lloyd then arrives at Mary's house and informs her he has her briefcase. He takes her to the hotel and shows her the briefcase and confesses his love after some initial struggle, but is rejected. Nicholas Andre, an old friend of the Swansons and the mastermind behind Bobby's kidnapping, arrives with Shay and, upon learning most of the ransom money has been spent by Harry and Lloyd and replaced with IOUs, takes Lloyd and Mary hostage, as well as Harry when he returns. Before Nicholas can kill them, an FBI team raids the suite and arrests him and Shay. After the incident, Mary and Bobby are reunited, much to Lloyd's jealousy, in which he fantasies of shooting him dead when he realizes he came all this way for nothing.
The next day, Harry and Lloyd begin walking home. All of the items they bought were confiscated and their moped has broken down. The two unknowingly decline the chance to be oil boys for a group of bikini girls, after which Harry ironically tells Lloyd that they will get their "break" one day. Harry and Lloyd then play a friendly game of tag as they continue to walk back home.

Question: How do Harry and Lloyd accidentally kill Mental?
Answer: with rat poison pills Mental tried to use on them
Alternative answer: With Rat Poison