I've tried to come up with ideas for 5e skill challenges to represent secret spy missions.
Secret Missions
While freeform ingenuity is part of what makes a spy mission or heist so exciting, it can be useful for the game master and players to have a general structure for how to resolve these scenes.
First the game master designs the mission by determining a variety of obstacles. Then the player characters have a chance to assess the challenge and prepare for the mission. Finally, for each obstacle they must devise a way to overcome it and achieve their objective.
The skill challenge has one phase for the game master – design the mission – and two phases for the players – before the mission and during the mission.
Design the Mission
To design a secret mission as a skill challenge, the game master should come up with a number of obstacles – usually at least two, but not more than the number of PCs. Additionally, players have the option to make things more complicated for themselves by adding obstacles, such as trying to frame someone else for whatever damage they cause. The more obstacles, of course, the more likely the party is to fail the mission.
The game master decides which obstacles will be apparent to the party, and which are hidden. For example, during a museum heist, it might not take the party any special effort to learn the layout of the museum, but the nature of any alarms would be hidden.
Similarly, while most obstacles must be overcome in the moment, some obstacles can be dealt with before the beginning the mission. For example, if the party’s stealing a relic only on display during a museum gala, they might need an invitation to get in. A character could simply try to smooth-talk their way in, but forging or stealing an invitation in advance could be a safer option.
Coming up with good obstacles requires a bit of finesse. Sometimes the group will all cooperate to prepare for the mission, but only send one character in. Other times they’ll be working in different places to bypass multiple challenges simultaneously. Either is fine.
Ability Check DCs. A typical obstacle requires a DC 10 check, but if a PC attempts something that’s suboptimal, this might increase the DC by 2. For instance, it’s easier to acquire an invitation to a gala in advance (DC 10) than to bluff your way in without one (DC 12). Likewise it’s easier to bypass various security doors in a museum when you’re there (DC 10) than to figure out what keys you need in advance (DC 12). But sometimes the party has no choice, like if the group’s locksmith can’t attend the gala.
Before the Mission
After the game master has designed the mission, they narrate what the player characters know about the mission and its non-hidden obstacles. Then each PC can undertake one effort before the mission starts. The player narrates what they’re doing and makes an ability check.
The most common efforts before the mission are to surveil and to prepare. Surveilling gets more information, which can reveal hidden obstacles or provide advantage in dealing with one of the obstacles. Preparing attempts to overcome an obstacle in advance, usually by acquiring the right tools – costumes, keys, passports, vehicles – or setting up favorable conditions – scouting an escape route, sabotaging an alarm, or getting the guards on your side.
Surveillance. A character looks for more information about the mission site, or the people involved, and then makes a skill check, typically DC 10.
They might try to examine a secure facility’s defenses up close while looking innocuous with Charisma (Stealth), or gather rumors about a gala by hobnobbing in high society with Charisma (Investigation). A long stakeout to learn guard patterns could be a Constitution (Perception) check.
On a success, first, the game master should provide a clear hint about whether there are any hidden obstacles in the mission. Then the character chooses one of the known obstacles and grants advantage to any checks that are made to overcome that obstacle.
If a second character succeeds in surveilling, they learn thoroughly about any hidden obstacles, and they choose a known obstacle and grant advantage to checks to overcome it.
If a character fails, they can simply accept the failure, or they can try again, but this increases the DC by 5. For instance, they might try to gather rumors, but when no one talks they could try being more explicit, which puts people on guard. If they fail a second time, they suffer some a complication (see below).
Preparation. A character chooses a known obstacle and narrates how they intend to solve it in advance, then makes a skill check.
They might craft costumes with Intelligence check with weaver’s tools, purchase fake passports with Charisma (Deception), steal keys with Dexterity (Sleight of Hand), or position a sailboat for a quick getaway with Wisdom check with water vehicles.
Usually the game master rolls in secret to determine how good the character’s preparations are, and the character cannot try again. Sometimes the character will know if they fail (such as if they try to steal keys) and, just like with surveillance, they can try again, but the DC increases by 5, and if they fail a second time they suffer a complication.
If they succeed, the group will find out during the mission when someone tries to rely on the preparation to overcome an obstacle. If they fail, the character who’s active in the mission probably gets caught with their pants down, but that doesn’t mean they’ve failed. They can still try and improvise a solution, but this increases the DC by 5.
During the Mission
After each member of the party has taken an effort, the game master and players collaboratively narrate how they approach the obstacles of the mission. For each obstacle, one character takes the lead in overcoming it. They describe what they’re doing, and if necessary they’ll roll a check to determine whether they succeed.
If the character fails, they have the choice to either abandon the effort, which usually means the mission has failed, or to try again, which increases the DC by 5. If they fail on their second check against a given obstacle, they suffer a complication.
Some obstacles need to be dealt with sequentially, and so all PCs could be present, but still only one character is taking the lead. At the game master’s discretion it might be reasonable for other characters to assist somehow and grant advantage, but that’s not always viable. In other situations the party will need to handle multiple challenges at the same time, forcing them to split up.
Deflecting Blame. A common player-imposed obstacle will be deflecting the blame of your actions to frame someone else. A character who takes the lead in this task narrates their plan, and then if necessary makes a check, usually against DC 10. Usually a character gets only one chance at this, and cannot try again if they fail.
Typically an effort to deflect blame must be done during the mission, but sometimes it can be done before the mission. If trying to frame a priest for defacing a rival god’s temple, they might scrawl appropriate quotes from scripture with Intelligence (Religion). To make the public suspect the head of the constabulary committed a murder, before they undertake their assassination they might print false threats of his toward the victim using Intelligence (Deception), or even make those threats in person with Charisma (Disguise Kit).
Complications
Complications come if a character fails two attempts of the same action during a secret mission. The most common sort is that the character’s efforts leave a clue that – usually after the mission is over – can direct investigators toward the party. Sometimes the clue can be rather blatant and can cause people to immediately view the character as a threat.
For example, if during a museum gala heist a character tries to cause a distraction so that the guests and guards aren’t looking while another ally snatches the relic, two failures means at least some people in the crowd figure out what the character was up to. If, though, the character trying to bypass the alarm on the case holding the relic fails twice, they might have to just smash it open, which immediately summons the guards.
The game master should try to make sure a complication isn’t automatically a failure. The party might defeat a few guards in battle, which cows the rest for a moment, letting the characters flee. Or they might just need to overcome a new obstacle, and make an ability check (now with disadvantage) to outrun the guards and avoid a fight.
Another complication might be a loss of face, such as if a character is surveilling by hobnobbing at a fancy party and is seen as gauche for asking too-pointed questions. A complication could cause a character to suffer a level of exhaustion, or damage (something between 1d6 and 1d10 per level). As always make sure any mechanical penalty is justified by the narrative and story.