This page of the Marvel's Guardians of the Galaxy game guide explains how to unlock the final, fourth ability of the Star-Lord, Gamora, Rocket, Groot and Drax. This could be vital knowledge when it comes to using all the skills available to the Guardians of the Galaxy.
In the individual team members' ability windows, you might notice that initially only 3 of their 4 abilities are displayed. The last ability is marked with a padlock icon and question marks. This indicates that it cannot be purchased with ability points.
Fortunately, the solution to this problem is very simple. These unavailable fourth skills are called mega abilities. They can't be purchased, because they are automatically unlocked at specific points of the campaign:
- The Star-Lord's mega ability is unlocked in chapter 10.
- Rocket's mega ability is unlocked in chapter 11.
- Drax's mega ability is unlocked in chapter 12.
- Gamora's mega ability is unlocked in chapter 12.
- Groot's mega ability is unlocked in chapter 14.
It's worth noting that the mega abilities will be unlocked even if you haven't yet purchased the given character's previous abilities. They aren't required to obtain mega abilities.