If you're looking for alternatives to using the term "spy mission" in your writing, consider using phrases such as covert operation, secret assignment, intelligence gathering mission or clandestine task. Other synonyms to consider include surveillance mission, undercover operation, reconnaissance mission or investigation assignment. Using different words to describe a similar concept can add variety and interest to your writing, and can help avoid repetition. You may also want to consider your audience and the tone you want to convey, as some of these synonyms may be more appropriate in certain situations than others.