In The Hangover Part III , the plot shifts away from the "morning after amnesia" formula of the first two movies. Instead, it’s an action-comedy chase film. The plot kicks off when the Wolfpack (Phil, Stu, and Alan) are carjacked by Marshall (John Goodman) and his goons.

“Is Doug in Hangover 3?”

Sam looked back at the screen. He could feel the algorithm breathing down his neck, ready to suggest “Is Doug in Hangover 4?” — a movie that didn’t exist, a sequel to nothing. But he didn’t need another sequel. He needed someone to ask the real question.

While the finale of the trilogy shifts away from the "blackout" formula of the first two films, it maintains the tradition of Doug being the catalyst for the adventure rather than a full participant in the chaos. Doug’s Role in The Hangover Part III

If you rewatch The Hangover trilogy, a very strange pattern emerges with Justin Bartha’s character, Doug Billings. While he is the catalyst for the entire franchise—getting lost on the roof in the first movie sets the entire chaotic tone—by the time we reach The Hangover Part III , Doug has essentially faded into the background.