In Untitled Mystery, Raghav, a banking employee near retirement, is taken to a gallery by his colleague Sharath, unaware that his friends are preparing a surprise birthday party at his home. As the party preparations unfold, his friends—Thomas, Hareesh, Shweta, Anoop, and wife Ambika—realize they all share a mysterious connection to Sharath, who has vanished from several different professional settings over the past five years. Each person recalls a distinct encounter with Sharath, from a naval base incident to an archaeological dig and even a medical surgery. As suspicions mount, the group is left questioning whether Sharath is who he appears to be. As Raghav's return draws near, the group must confront their shared suspicions about his connection to Sharath, culminating in a tense moment at the door.