As you zoom past an AT-AT's legs, Luke tells you exactly when to fire the harpoon. You must wind your way around the AT-AT while the cable pays out, being careful not to get too far away, and also not to hit the beast's legs or get shot by its lasers. Wing around and finish it off with your guns. The missions are generally quite challenging, with multiple objectives. This backfires occasionally, such as on a long mission on Corellia with a tough ending.
Failure means having to spend twenty minutes getting to that point again, which becomes tedious.
On the other hand, the mission to destroy some Imperial construction yards on Balmorra is so darn fun, constant replay is welcome. Oh, there's a story that links the missions, but it's not very important. Completing the basic 16 missions unlocks bonus levels, including the Battle of Hoth and the Death Star Trench. You can also fly a skyhopper through Beggar's Canyon on Tatooine. I was on the lookout for womp rats, thought I saw one and opened fire, but it turned out to be a land speeder.