GMs, we need to have a talk...
1. You don't get to force the PCs to be taken captive.
In most RPGs, the only (or at least primary) way your players have to tell their part of the story is through their characters' actions. By forcing them to become captives, you take away that ability to tell the story.
Being imprisoned isn't the problem. There are whole RPGs about being imprisoned (cf Dream Park, 1992). The transition from free to imprisoned is the problem. Just like you can't force the players to go into the dungeon, you can't force the players to travel to Paris, and you can't force the players to walk into the spooky haunted castle, you can't force the players to go into captivity.
2. You can get the players' buy-in, though
You can ask the players, out of character, if they agree to a situation where they get taken captive. If they trust you, and you make it sound fun, they'll agree. If they agree, be nice about logistical things like letting them getting their stuff back, or else they won't trust you as much in the future.
If they don't agree, find out why. Maybe they want a fair shot at escape. Maybe they believe their characters would rather die than be captured by the people you suggest they get captured by. Ask them under what circumstances their characters would find themselves prisoners. Allow them to tell a story about how they got captured that's comfortable to them.
Example: "I would rather die than let myself be captured by vampires. My character has a terrible fear of being bitten by a vampire. But if they hit me with a high level Sleep spell of high enough level, they could tie me up and drag me off before I could resist."
Don't think of it as "that player is trying to tell me what to do." Think of it as "she just gave me a cool new henchman for the vampire queen -- a sneaky wizard!"
3. Foreseen stakes
Players will accept being captured as the consequences of a die roll they miss, but only with all of the following conditions:
- You presented the stakes before the roll
- The roll appears fair
- The failure stakes sound fun
- You have their trust
For instance, in a D&D game, the party is camping in the wilderness. The Dragonborn Sorcerer is on watch. You tell the players, "Some drow are coming to capture you four. If they capture you, it'll be a cool escape quest. If not, you'll have a drow warband to investigate from the outside. Either way, I'm sure it'll be fun. They're going to use their poison arrows on the Sorcerer, and put their drow poison in your mouths while you sleep. Each of you gets to make either a DC 13 Constitution save or Perception check -- your choice. Everyone has Disadvantage because they're sleeping, except the Sorcerer. If two of you succeed, you fight off the drow, and they retreat. If less than two of you succeed, you've been captured. There are too many drow for just one of you to fight off. OK?"
This example follows the rules:
- Everything was explained before the players rolled, so they know what happens if they fail
- The roll appears fair - more than fair, really. If the sorcerer got hit with multiple arrows, she'd have to make multiple saving throws.
- The GM explained how both success and failure on the group check would be fun. This should always be true: Never call for a roll where either success or failure is boring!
- We're assuming the players don't have a problem trusting this DM
4. If you don't talk to them about it ahead, it feels like railroading
Let's say you decide to capture the PCs by using a really powerful encounter where the NPCs use nonlethal attacks to capture the party; but you don't present the stakes before the combat, and you don't explain how being captured could be fun in this case.
This is going to seem like railroading, because while any encounter is technically winnable if the players' dice come up 20s every roll, in reality, that's not true. Using overwhelming opponents and being a jerk about retreat/escape will cost you a lot of trust. After you burn your players' trust, do you really think they're going to be excited about the "you've been captured" adventure? Even your best friend will have their enthusiasm dulled a little by the forced capture.
Here's why it feels so bad: The social contract of most RPG battles is that every battle is technically winnable unless you're attacking something you know is way too powerful for you. Level 1 D&D characters can't take down a Lich, but the DM will tell them they can't take down a Lich before they try it. They won't be ambushed by a Lich on the way to the castle. While it's common to put overwhelmingly powerful monsters in your world, it's lame to put overwhelmingly powerful monsters in your world and then not tell the players they're overwhelmingly powerful. And it's really crappy GM behavior to ambush the PCs with overwhelming monsters they can't escape from.
5. A truly skilled GM knows how to make them let themselves be captured
Why do all the work? Why not make the PCs figure out how to get themselves captured?
James Bond always walks right in to his opponent's den, and he's almost always captured as a result. And he always profits from it!
Why does he do this? He could assault the enemy's fortification or try to sneak in. He's good at both approaches. But both involve a lot of risks, including the risk of death. Letting himself get captured often reveals information -- not just "before I kill you, Mr. Bond," monologues, but the layout of the site, the location of things he needs, the relationship between the henchman and the villain, and so forth. In addition, brazen moves that get him captured usually force the villain to make mistakes, panic, recall henchmen, postpone executions of people Bond wants to save, or abandon additional plans in favor of haste.
If you want your players to start thinking of getting captured as a victory, you just have to make getting captured the most expedient plan. It helps if you tell the players that...
- ...if the enemy force gets them, they won't kill them, and there will be ample opportunities to escape.
- ...the villain will likely make specific mistakes if they put themselves in the villain's hands ("MI-6 is on to us? We need to accelerate the timetable!").
- ...other approaches are very dangerous - give them reasons why stealth, assault, and disguise put them at more risk than getting captured.
- ...being captured is the best starting point for a stealth, assault, or disguise plan. In fact, they can prepare for capture, smuggling in lockpicks and such.
Still, the protagonists letting themselves get captured is different from the protagonists being captured against their will, and you might want both sorts of scenes.
6. Use the system if you can
Only a few RPGs have a mechanic for bribing the PCs to let something bad happen to them. Fate has compels. The Cipher System has GM intrusions. There aren't that many. If you have access to this tool, use it! In Fate, for instance, you can use a compel to offer the PCs a Fate Point (meta-game currency they can use for bonuses or doing their own compels) for getting captured. The can refuse the compel, spending a Fate point instead, and describe how they escape. It's not perfect, but it's a much smoother way to handle it than a die roll!