When “Mokey Mokey King” that was Special Summoned by “Magical Scientist” is returned to the Fusion Deck, it has been removed from the field so its effect activates and you Special Summon as many “Mokey Mokey” as possible from your Graveyard.
Missing the Timing: If “Mokey Mokey King” leaves the field in the middle of a chain (for example, if destroyed by “Ring of Destruction” chained to “Pot of Greed”, or Tributed for a Tribute Summon or for the effect of “Cannon Soldier”, etc.,), you miss the timing and you cannot Special Summon. Only when the last action of the chain is the removal of “Mokey Mokey King” from the field, can you activate its effect.