Complex sentence: After a phone call with Carrie, Quinn assumes that she is going to refuse his proposal, so he accepts the mission just as the assassination team is about to leave, after having written his own letter addressed to Carrie.
Split into simpler sentences: After a phone call with Carrie, Quinn assumes that she is going to refuse his proposal. He accepts the mission just as the assassination team is about to leave, after having written his own letter, addressed to Carrie.