The first mystery
The first level. Ashley, who's wearing green, must flip the switch to open the cage so Mary Kate can collect the puzzle piece.
And She does so.
The puzzle pieces a few levels in.
Ashley throws Clue at a cat
Mary-Kate gives Ashely a boost to a switch
The levels get more complicated farther in. This one has three switches and is two screens wide.
Title screen on black & white Game Boy
It's difficult to tell Mary-Kate and Ashley apart on a black & white system.