There are only a few flash games which are based on the story instead of the game play and one of them is Sift Heads World Act 6. This game is one of the most popular assassin games around.
You will have to finish dozens of missions if you want to reach the end, but each one of them will be completely different from the other. After you receive the briefing, you will have to pick a weapon and try to assassinate your enemies. If you have to infiltrate a building, you can move your mouse towards the sides of the screen and click on the arrows. You can use the menu for choosing a different weapon or for taking another look at the mission briefing. Once you complete the objectives, the mission will automatically end and you will be sent back to your hideout. Here you can use your computer or your regular mail for getting other missions. Your next job will start whenever you want and sometimes you will have more than one choice. Sift Heads World Act 6 is not a complicated game as long as you pay attention to what's going on. If you forget your objectives, you will have to retry the mission because one wrong move can end the level.
When it comes to assassin games, Sift Heads World Act 6 is one of the most complicated ones that you can find online. It has drama, action and dark comedy, so you can be sure that you won't get bored.