Sometimes the players change the plot

A player is bringing in a new character to the game. The group is pledged to the local King through their artifact weapons. In order to unleash the power of the weapon, one must become 'soul bound' to it. In becoming 'soul bound' a character also gains a bond to the other weapon wielders. Certain powers of the weapons are additive if more than one weapon is being used along side another. The powers of evil, for I like having powers of evil in my game, recognize the artifact weapons as a threat and therefore desire to prevent another weapon gaining a 'soul bound' wielder. The player's new character manages to avoid the trap laid by the powers of evil, meets her new comrades, and prepares for the binding ceremony the next morning. Lots of pomp and circumstance, some noble feel good moments and then off to the grand adventure I had planned for them. But not so fast, for the players had other ideas. As I mentioned before, I prefer a Story Telling Adventure Game type of play. So I have a strong story arc, and the players work within that to tell the story. You see it was the night before the ceremony and the players assumed that I would be a Rat Bastard DM and attempt another assassination attempt on the new character. There was a precedent, as the powers of evil have used assassination in the past and will most certainly do so again in the future. So as the players began to fret a bit and plan against the attempt, I let the paranoia build. As any good DM will tell you, some paranoia is good, and any paranoia is fun (at least for the DM). However, I let it get out of hand and the players spent a goodly part of the evening preparing for the assassination attempt that was not in the story arc. Letting the night pass without the anticipated event would be disappointing to the players, and the time left for our game evening would only allowed me to do the ceremony and not much else. So I was left with a choice - stick to my story arc or improvise. I hate to let an evening pass without the players having some good dice rolling and excitement, so improvise it was. Why not give them what they want? I bought a little time by saying, "OK, draw out the inn and show me where everyone is going to be." I ran through the options in my head, altered a villain slightly and had that said villain send a leveled invisible stalker after the new character. The battle turned out quite memorable, with characters going down, characters risking their lives to protect others, some clever actions and a humorous moment when the wizard blasted a hole through the wall on the second floor of the inn and destroyed the roof of the neighboring building. All in all a good nights work. In the morning they had the ceremony but not before the innkeeper told them they were no longer welcome at his inn. Heroism has its price. I do not always let the players write the story arc, but sometimes the story is enriched by letting them do so. Turns out the changes to the villain opened up a number of interesting possibilities that I have already used in the game to the general benefit of the future story. Now to be clear, the players/characters ALWAYS have a hand in the story. The plot outline is written but the outcome of events is not determined by me but by their actions. And of course their actions have consequences, good and bad, which I allow to play out. So though I am the primary creator of the plot outline, the players give it life and alter its direction. Sure this has some challenges, but overall it keeps the game fresh, unpredictable, and exciting. The dice never lie.