Complex sentence: In The Avengers a ghostly Marissa appears to Summer, when she returns to the adjoining bedrooms they shared together, for the first time since Marissa's death, this causes Summer to fast track her trip back to Brown and leave Newport.
Split into simpler sentences: In The Avengers Summer believes she sees Marissa-as a ghost-when she returns to the adjoining bedrooms they shared together for the first time since Marissa's death. This causes Summer to fast track her trip back to Brown and leave Newport.