The plot of Psych 3: This Is Gus follows the main characters, Shawn and Gus, as they prepare for a shotgun wedding to Selene, who is pregnant with Baby Guster. In the midst of their preparations, they learn that Selene's estranged husband is still alive and may try to interfere with their plans.
Determined to track down the husband and prevent any disruption to their wedding, Shawn and Gus go rogue. Their investigation leads them to uncover secrets about Selene's past and the reasons for the estrangement from her husband. Along the way, they encounter danger and cryptic clues that test their detective skills and their friendship.
Meanwhile, Lassiter (played by Timothy Omundson), who has recently been reinstated to the police force after recovering from a gunshot wound, faces new challenges in his career. As he grapples with the future of his job, he finds himself drawn into Shawn and Gus's investigation and their race against time to ensure a happy ending for Selene and the Guster family.
The movie is a continuation of the popular TV series Psych, which aired for eight seasons on the USA Network. It features the return of fan-favorite characters and the same quirky humor and clever detective work that made the original show a hit.