This takes place after the ninjas save Ninjago from the overlord which resulted in some of them losing there powers for the time being. Master Wu knew the day their powers would return to Ninjago and once more would it be in danger so he decided to put the Ninjas through intense training and tests to find their biggest strengths and weaknesses. The main weakness for the ninjas was their environment as long as they were surrounded by their element they were able to pull out the most in there powers but if there environment didn't help supply there elemental powers they were weak. Master Wu needed to fix that.