Alright, let me tell you what I know about DMing, and why I put so much work into setting, so you know how I think when we make this.
Here's the problem, is that without a detailed setting, your game world is empty. Look at knights academy. I have a town with multiple locations, and a world map set up, and we haven't even seen it all. There's also a basic history and multiple, diverse NPC inhabitants. Making a detailed setting like that is hard, but it pays off because the world feels more real.
Second thing to know, is that you need a goal. If the players have something to work towards, they'll be driven to continue playing. All my players have at least one goal: to graduate and become a knight, in addition to any personal goals they had when writing their character. This big group goal also encourages them to work together as a team. What's more, you need the goal and setting to work together as one. Do not add a fantasy based goal to a modern setting. Make sure it makes sense within the world. Make sure your players all have valid reason to want to complete this goal.
Lastly, a few things I keep in mind while DMing. Firstly, do not talk about your plans to the players. They won't want the story spoiled, and it makes them feel like they're being spoon fed, like a baby. Don't tell them what they should do OOC, at best you should guide them with NPCs, notes, and the like. Do not, EVER, choose one player as a protagonist. If you show favor to a single player, the others will get bored and leave. Every player should have equal opportunity to be the hero.
But I digress. I'll help you with your plot, but you're going to need to answer a few questions. Firstly, what genre shall the game be?