The shortest flight route is not always a straight line. We are used to see the Earth represented flat in a two-dimensional map, however this representation ignores that the Earth is round. The curvature of the Earth means that the most direct routes between Europe and North America or between Europe and Asia would normally go across the polar regions, since the distance to be covered is smaller at those places. This is known as the Great Circle Route.
If you want to see what is the most direct route between two points check out this website
For example, for the Lisbon to Tokyo (Narita airport) example enter LIS-NRT and press "Map", you will be surpised by the results! You can do the same with any airport city codes (you can find a list of airport codes here.
Now you can see why Finnair is focusing on marketing its Helsinki hub (HEL) as the gateway to Asia...