Complex sentence: D'Artagnan and his three friends volunteer to travel to England to retrieve the jewels, but along the way, they are ambushed by the Cardinal's men and one by one, they are forced to stay behind to hold off their pursuers.
Split into simpler sentences: D'Artagnan and his three friends volunteer to travel to England to retrieve the jewels, but along the way, they are ambushed by the Cardinal's men. One by one, the Musketeers are forced to stay behind to hold off their pursuers.