Well the obvious parts is something tragic happened. No one who was there wants to bring it up. What could have caused his amnesia is still a mystery. Could be head trauma or it could be emotional trauma. Considering he had to stay home and recover I am going with something physical. / I do find the ~8 year age difference a bit weird between Walter and the non student cast. However that could be another clue to what happened in his past considering not many situations come up where kids and teens are just eft unattended and could get hurt. Perhaps a kind of boy scouts or camping trip? / I believe we might get more info on days 9 and 10 especially since the beach trip has lead to Walter uncovering more about his past and repressed memories resurfacing to the point where he is now aware this might not be the first time he has met this cast of friends. Be patient. More will be revealed soon enough.