There are a number of generally useful techniques that are worth looking at, but of course a lot of it depends on how they went off course.

• They made an enemy of someone that needs to be an ally This happens in real life and fiction all the time. One of the most common resolutions is to find a way to work together anyway because there is a need, at least temporarily. The tension can in fact create a lot of interesting dynamics. Drizzt worked with Artemis Entreri more than once because there was a temporary pressing need. In the Star Wars EU, the New Republic and the Remenants of the Empire put aside their difference temporarily during the Yuuzhan Vong war. You might have to ramp up the tension to drive them together, but almost any difference can be put aside at least temporarily if the need is great enough.
• Killed someone crucial Well, I know you said you don't like "Don't let it come to that" but there is almost always some way to keep someone truly vital alive. They have a cunning escape route preplanned. If you are in a game system (D&D) that allows easy resurrection, they get resurrected. If you do decide it's better to let them die, then replacing them (depending on the situation) might not be so hard. The NPC might have a henchmen/son/daughter/etc. that is ready and willing to continue on the original person's schemes. They would likely know most of the stuff the original knew, and might have the original's notes, records, or (depending on setting) even access to the dead man's soul.
• Rejected the job If they turn down some sort of job your plan required them to take then you can do a number of things to get them more interested. The most obvious is to up the offered reward, but you could also do something to directly force them. For instance, if the job was to kill someone and they refused, you could later have that someone kill one of their loved ones, and then its personal. Perhaps the people trying to hire them decide they need to gauruntee compliance by taking a captive. If you really want to be ham handed, they could be put under some sort of geas/implanted with a mind control device that directly forces them to take it.
• Lost an essential item If they lose an essential item, then it might find its way back to them on its own. This is a common trope with magical items, especially cursed ones. But a better way is to reinforce its importance to them and then give them an opportunity to earn it back. This can create its own interesting twists and might become a "MacGuffin".

Now, one thing that I find helps when my plans as a GM get derailed is to let time pass, both in real life and in the game. This gives me time to formulate exactly how to go forward and in game it gives time for the setting to adjust. They killed the main villain too early, I can easily replace him with his own son, but it seems funny that the fully-ready kid is just standing there ready to take up the slack. It seems a lot less funny that the kid was out on his own adventures, but was being kept aprised of the situation by his father. The father dies, the kid travels back to the action and then takes up his place.

The best way to let time pass is to give them some sort of side mission and then come back to the main plot.

Of course, if they really mess up your plans it might be important to make sure the players liked that plot. If they did something reall unexpected that ruined the plot, it might be a sign that they don't like that plot and you as the GM should be makings ome major tweaks anyway.

There are a number of generally useful techniques that are worth looking at, but of course a lot of it depends on how they went off course.

• They made an enemy of someone that needs to be an ally This happens in real life and fiction all the time. One of the most common resolutions is to find a way to work together anyway because there is a need, at least temporarily. The tension can in fact create a lot of interesting dynamics. Drizzt worked with Artemis Entreri more than once because there was a temporary pressing need. In the Star Wars EU, the New Republic and the Remenants of the Empire put aside their difference temporarily during the Yuuzhan Vong war. You might have to ramp up the tension to drive them together, but almost any difference can be put aside at least temporarily if the need is great enough.
• Killed someone crucial Well, I know you said you don't like "Don't let it come to that" but there is almost always some way to keep someone truly vital alive. They have a cunning escape route preplanned. If you are in a game system (D&D) that allows easy resurrection, they get resurrected. If you do decide it's better to let them die, then replacing them (depending on the situation) might not be so hard. The NPC might have a henchmen/son/daughter/etc. that is ready and willing to continue on the original person's schemes. They would likely know most of the stuff the original knew, and might have the original's notes, records, or (depending on setting) even access to the dead man's soul.
• Rejected the job If they turn down some sort of job your plan required them to take then you can do a number of things to get them more interested. The most obvious is to up the offered reward, but you could also do something to directly force them. For instance, if the job was to kill someone and they refused, you could later have that someone kill one of their loved ones, and then its personal. Perhaps the people trying to hire them decide they need to gauruntee compliance by taking a captive. If you really want to be ham handed, they could be put under some sort of geas/implanted with a mind control device that directly forces them to take it.
• Lost an essential item If they lose an essential item, then it might find its way back to them on its own. This is a common trope with magical items, especially cursed ones. But a better way is to reinforce its importance to them and then give them an opportunity to earn it back. This can create its own interesting twists and might become a "MacGuffin".

Now, one thing that I find helps when my plans as a GM get derailed is to let time pass, both in real life and in the game. This gives me time to formulate exactly how to go forward and in game it gives time for the setting to adjust. They killed the main villain too early, I can easily replace him with his own son, but it seems funny that the fully-ready kid is just standing there ready to take up the slack. It seems a lot less funny that the kid was out on his own adventures, but was being kept aprised of the situation by his father. The father dies, the kid travels back to the action and then takes up his place.

The best way to let time pass is to give them some sort of side mission and then come back to the main plot.

Of course, if they really mess up your plans it might be important to make sure the players liked that plot. If they did something really unexpected that ruined the plot, it might be a sign that they don't like that plot and you as the GM should be makings ome major tweaks anyway.

There are a number of generally useful techniques that are worth looking at, but of course a lot of it depends on how they went off course.

• They made an enemy of someone that needs to be an ally This happens in real life and fiction all the time. One of the most common resolutions is to find a way to work together anyway because there is a need, at least temporarily. The tension can in fact create a lot of interesting dynamics. Drizzt worked with Artemis Entrerir more than once because there was a temporary pressing need. In the Star Wars EU, the New Republic and the Remenants of the Empire put aside their difference temporarily during the Yuuzhan Vong war. You might have to ramp up the tension to drive them together, but almost any difference can be put aside at least temporarily if the need is great enough.
• Killed someone crucial Well, I know you said you don't like "Don't let it come to that" but there is almost always some way to keep someone truly vital alive. They have a cunning escape route preplanned. If you are in a game system (D&D) that allows easy resurrection, they get resurrected. If you do decide it's better to let them die, then replacing them (depending on the situation) might not be so hard. The NPC might have a henchmen/son/daughter/etc. that is ready and willing to continue on the original persons schemes. They would likely know most of the stuff the original knew, and might have the original's notes, records, or (depending on setting) even access to the dead man's soul.
• Rejected the job If they turn down some sort of job your plan required them to take then you can do a number of things to get them more interested. The most obvious is to up the offered reward, but you could also do something to directly force them. For instance, if the job was to kill someone and they refused, you could later have that someone kill one of their loved ones, and then its personal. Perhaps the people trying to hire them decide they need to gauruntee compliance by taking a captive. If you really want to be ham handed, they could be put under some sort of geas/implanted with a mind control device that directly forces them to take it.
• Lost an essential item If they lose an essential item, then it might find its way back to them on its own. This is a common trope with magical items, especially cursed ones. But a better way is to reinforce its importance to them and then give them an opportunity to earn it back. This can create its own interesting twists and might become a "MacGuffin".

Now, one thing that I find helps when my plans as a GM get derailed is to let time pass, both in real life and in the game. This gives me time to formulate exactly how to go forward and in game it gives time for the setting to adjust. They killed the main villain to early, I can easily replace him with his own son, but it seems funny that the fully ready kid is just standing there ready to take up the slack. It seems a lot less funny that the kid was out on his own adventures, but was being kept aprised of the situation by his father. The father dies, the kid travels back to the action and then takes up his place.

The best way to let time pass is to give them some sort of side mission and then come back to the main plot.

Of course, if they really mess up your plans it might be import to make sure the players liked that plot. If they did something reall unexpected that ruined the plot, it might be a sign that they don't like that plot and you as the GM should be makings ome major tweaks anyway.

There are a number of generally useful techniques that are worth looking at, but of course a lot of it depends on how they went off course.

• They made an enemy of someone that needs to be an ally This happens in real life and fiction all the time. One of the most common resolutions is to find a way to work together anyway because there is a need, at least temporarily. The tension can in fact create a lot of interesting dynamics. Drizzt worked with Artemis Entreri more than once because there was a temporary pressing need. In the Star Wars EU, the New Republic and the Remenants of the Empire put aside their difference temporarily during the Yuuzhan Vong war. You might have to ramp up the tension to drive them together, but almost any difference can be put aside at least temporarily if the need is great enough.
• Killed someone crucial Well, I know you said you don't like "Don't let it come to that" but there is almost always some way to keep someone truly vital alive. They have a cunning escape route preplanned. If you are in a game system (D&D) that allows easy resurrection, they get resurrected. If you do decide it's better to let them die, then replacing them (depending on the situation) might not be so hard. The NPC might have a henchmen/son/daughter/etc. that is ready and willing to continue on the original person's schemes. They would likely know most of the stuff the original knew, and might have the original's notes, records, or (depending on setting) even access to the dead man's soul.
• Rejected the job If they turn down some sort of job your plan required them to take then you can do a number of things to get them more interested. The most obvious is to up the offered reward, but you could also do something to directly force them. For instance, if the job was to kill someone and they refused, you could later have that someone kill one of their loved ones, and then its personal. Perhaps the people trying to hire them decide they need to gauruntee compliance by taking a captive. If you really want to be ham handed, they could be put under some sort of geas/implanted with a mind control device that directly forces them to take it.
• Lost an essential item If they lose an essential item, then it might find its way back to them on its own. This is a common trope with magical items, especially cursed ones. But a better way is to reinforce its importance to them and then give them an opportunity to earn it back. This can create its own interesting twists and might become a "MacGuffin".

Now, one thing that I find helps when my plans as a GM get derailed is to let time pass, both in real life and in the game. This gives me time to formulate exactly how to go forward and in game it gives time for the setting to adjust. They killed the main villain too early, I can easily replace him with his own son, but it seems funny that the fully-ready kid is just standing there ready to take up the slack. It seems a lot less funny that the kid was out on his own adventures, but was being kept aprised of the situation by his father. The father dies, the kid travels back to the action and then takes up his place.

The best way to let time pass is to give them some sort of side mission and then come back to the main plot.

Of course, if they really mess up your plans it might be important to make sure the players liked that plot. If they did something reall unexpected that ruined the plot, it might be a sign that they don't like that plot and you as the GM should be makings ome major tweaks anyway.