Complex sentence: Dredd travels to Oz, ostensibly to arrest Chopper and return him to Mega City One and imprisonment, however he is arrested as Chopper arrives in Oz and placed under house arrest.
Split into simpler sentences: Dredd travels to Oz, ostensibly to arrest Chopper and return him to Mega City One and imprisonment. However Dredd is arrested as Chopper arrives in Oz, and apparently placed under house arrest.