Luke Haldeman was an 09er and baseball player at Neptune High. He got caught up in doing and smuggling drugs, including steroids. He had to sell his signed baseball to pay back the drug dealer whose merchandise Troy Vandegraff stole on the way back from Tijuana. He also provided Dick Casablancas with the GHB that, though intended for Madison Sinclair, ended up drugging Veronica at Shelly Pomroy's party.
After graduation, he was with Dick, Carrie Bishop, Susan Knight, and Gia Goodman on a party boat trip when Susan died of an overdose. He helped Carrie and Gia throw her body overboard while Stu "Cobb" Cobbler photographed them. Stu used the photo as leverage over them for years. When Carrie Bishop (now a pop star dubbed 'Bonnie DeVille') became overwhelmed by the guilt of the cover-up, she attempted to confess her crimes through her music. Cobb ordered Luke and Gia to help him murder her, which he did by electrocuting her in her bathtub.
By the time the 10-year reunion came around, Gia and Luke were engaged to be married. However, Gia was revealed to have been simply a beard to cover up Luke's closeted homosexuality and Veronica uses this fact against Gia to reveal her affair with Cobb and unravel the lies behind Carrie Bishop's murder.