The setting is this: One day, his dad (MJF's Marty) comes up missing and his son can't find him. He figures out that he (MJF) had come into the son's lab one night and was messing with his wrist time-teleport experiments and got sucked 'somewhere' into time.
The son now is on a quest to find his dad (MJF) but his science is not there just yet. There was only enough power for two time trips, one to 'wherever' and one back. Accidentally, his dad took one of the trips, so if he goes looking to find him, he has no way of knowing where exactly he is or how to get them both back back if he did find him. (MJF had picked the device up, looking at it and accidentally triggered it, but was not wearing it so that only he got sent, 'somewhere' dropping the device, thus leaving it behind)
But, he remembers the stories fondly that his dad used to tell about Doc Brown! This crazy inventor who created the time travel in the first place.