The 5th edition Dungeon Master’s Guide dedicates an entire chapter to building an adventure out of encounters, but it has very little in the way of guidance as to how to structure this as a session to be played at the table. Some gaming groups go and go until at least one person breaks and they have to end the session there, but I always attempt to plan my sessions into discrete chunks that last around two to three hours.
For a typical D&D session, I plan for three combat scenes, two to four non-combat scenes, and one random encounter combat scene. When I say "combat scene,” I’m not saying it’s certain the players will fight something, but that I’m going to put them in a situation where an armed confrontation with a group of opponents of roughly equal combat ability is likely. They could still sneak past, parley, run away, or come up with all sorts of other ideas. The same goes for non-combat scenes. I may expect them to negotiate with the ornery shopkeeper, but they could also stab him and run off with the goods. If we assume about a half hour for combat and 15 minutes for non-combat, that’s right where I want the length to be.
Start With The End
I start the design process with the most important scene, the climax. Traditionally, this is either the “big boss fight,” or a non-combat scene leading up to or immediately after the biggest, hardest fight of the session. This will inform the rest of the planning, as everything else in the session should lead up to this moment where the players have a chance to reach their goal for the session. You don’t have a goal for your players to reach for this session? This is where you figure that out. This is often where I start looking at the “Monsters by Challenge Rating” chart. If I happen to see that a stone giant is an appropriately leveled boss for the session, that could inspire the theme of the session.
Research shows that we overvalue the ending of an experience when rating how enjoyable it was as a whole. Put your best work at the end and it will make everything else you did seem better. It also means that it’s good to have a climax that ends with the players accomplishing something significant. If you want to have the villains come out on top, I recommend figuring out a way to do that through a final non-combat scene.
Back To The Beginning
Armed with a rough draft of the climax, I jump forward to the first scene. This is where it’s made clear to the players what the goal for the session is. That means this is often a non-combat scene, but sometimes ninjas just show up and steal something, creating a goal of getting it back.
It’s good that this and the climax are the most important, because they are the scenes the DM has the most control over. That’s who has the power to set the stage and determine when the session is over. You can exercise a bit more narrative control before the session starts too. If a ninja sneaks into camp in the middle of play and steals something without making stealth/sleight of hand checks opposed by the party’s perception rolls, your players will call you a cheater, or worse, a railroader. However, they are much more forgiving of “You travel for several days down a dusty road without incident, passing only the occasional merchant, until one morning you awaken and find that the Ring of Destiny is missing. Ninja tracks lead into the forest…”
Fill The Middle
Now we come to the messy middle. Players can’t avoid the first scene since they’re dumped into it, and I’ve already all but told you to hold them hostage until they reach your climax, so leave the middle as open as possible. This is where the players should feel that they are in the driver’s seat, making the decisions that should bring them hurtling towards the end. A good model for this is a dungeon with one entrance, one goal, but many branching rooms and hallways in between. This obstacles crop up that prevent the players from reaching their goals. If they don’t go the way you were thinking and skip some of these scenes, that’s fine! We’re just trying to build to the climax.
As my list of scenes grows, I try as much as possible to alternate combat with non-combat. Again, players can and will skip scenes or take them out of order, but I think the ideal is to alternate. This keeps combat fresh, instead of feeling like a meat grinder. I also make my early combat scenes not too difficult, usually in the medium-hard difficulty range. I find that a couple easier fights will sap some of the party’s resources, making the final hard-deadly climax more interesting, but it won’t be hard enough that players feel the need to take a long rest in the middle of the adventure.
The final scene I plan is an optional random encounter. I use this as per novelist Raymond Chandler’s famous advice: "When in doubt, have a man come through a door with a gun in his hand.” It usually has little bearing on the plot, but I like to have an easy combat scene at the ready. If a scene is dragging, the players are talking themselves in circles about what to do next, or they’re just completely lost, I drop it in. This usually wakes everyone at the table up, and can be used to redirect them back into the story.
With these 6-8 scenes, you should have enough content to fill a satisfying gaming session. I left the more creative aspects of building encounters and adventures out, but I’m a big believer in putting structure first. I feel that three combats alternating with non-combat scenes is the perfect amount for keeping everyone at your table engaged throughout the entire session and leaving them wanting more.