Complex sentence: Billy and Eva tried to rescue Marta when the O'Driscolls were at a trade fair, breaking into the house and found Marta locked in the bedroom but were caught when the O'Driscolls returned early.
Split into simpler sentences: Billy and Eva put into effect a rescue attempt when the O'Driscolls were at a trade fair. They broke into their house and found Marta locked in the bedroom but were caught when the O'Driscolls returned early.