Complex sentence: Together Colin and Frederique use some stolen letters to track down who they believe to be the secret group Thirteen in a house where two different groups of actors are separately rehearsing for productions of Aeschylus's "Prometheus Bound" and "Seven Against Thebes".
Split into simpler sentences: Together Colin and Frederique use some stolen letters to track down who they believe to be the secret group Thirteen. This leads them to a house where two different groups of actors are separately rehearsing for productions of Aeschylus's "Prometheus Bound" and Aeschylus's "Seven Against Thebes".