Complex sentence: They retreat, but the kraang followed them, so they got away in the shellraiser, afterwhich Jack tells them that they need to infilitrate TCRI in order to erase all of April's DNA codes.
Split into simpler sentences: They retreat, but the Kraang followed them so they got away in the shellraiser. Afterwards, Jack tells them that they need to infiltrate TCRI in order to erase all of April's DNA codes.