Complex sentence: Austin O'Brien co-stars as a boy who is magically transported into a parallel universe by a magical movie ticket, one inhabited by Slater and the other characters in the "Slater" film series.
Split into simpler sentences: Austin O'Brien co-stars as a boy who is magically transported into a parallel universe by a magical movie ticket. Inhabited by Slater and the other characters in the "Slater" film series.