Ben 10: Power Trip

The first available side mission comes after you have been to Strudelbek City. Dorothy Hairball will ask you to find her cat Fluffy. Once you accept the mission, turn 180 degrees and the cat should be on the roof or the balcony of the building. Jump up and catch her. She may run and you will have to give a little chase. Press X when you are close and you should catch her. Once you have her you will be transported back to Ms. Hairball and the achievement will pop.

If you are like me and didn't realize there was a side mission, you can check by pressing the menu button and then Pressing the Left Stick over to the right once. You will now see any side missions that are available. If you are in the middle of a main mission, you will not be able to select a side mission until it is complete. For example, I had already talked to the scientist in the swamp and was tasked with collecting mushrooms, cattails, and truffles. I was not able to switch over to the side mission until I had completed this task and had delivered the wares.