Original: As soon as they arrive, Knuckles ambushes Sonic from underground with such force, he knocks the Chaos Emeralds from him, returning him to normal. Stunned, Sonic can only watch as Knuckles steals the Emeralds and disappears inland.
Simplified: However, Knuckles surprises them, and takes away the Chaos Emeralds Sonic and Tails have collected.