Let your guests guide the plotline of the night. With more guests, the plot can be thicker, and based on everyone's interests (or the occasion), some plotlines may be more fun to act out than others.
While you could concoct your own murder tale, and then write the scripts and flesh out the clue files for each "character," it’s a time-consuming process. Thankfully, there are several businesses that offer murder mystery packages. Merri Mysteries has a number of kits with clues and instructions — you just need the costumes, food, and friends. Others, like The Murder Mystery Co., will provide everything, from scripts to real actors in character.