The year is 1925. The Colonel's Bequest follows Laura Bow, a young student attending Tulane University in New Orleans, as she spends a weekend at her uncle's mansion on a secluded island. During dinner, she meets members of Lillian's extended family and their alleged friends, but soon discovers that old feuds and schemes are brewing. As strange accidents and murders occur, Laura must investigate to find the culprit.
The gameplay of The Colonel's Bequest is a detective mystery adventure game similar in style to Agatha Christie's novels. The player can explore the island, interacting with characters and observing their actions to uncover clues. The clock advances every quarter hour, giving the player time to think before acting. Characters have schedules, and some actions can be done using keyboard shortcuts.
The gameplay focuses on exploration, with much of the plot hidden in the background. Players can discover new information by talking to characters about various topics. There is an inventory and a few puzzles, but they are less prominent than in other Sierra games. The game does not have a scoring system, and players are given a detective ranking at the end, along with hints for any story branches or missed elements.