Complex sentence: On July 4, Levinson determines the key to defeating the aliens lies in deactivating their force fields, and devises a way to do so by uploading a computer virus into the mothership using the refurbished alien fighter, which Hiller volunteers to pilot.
Split into simpler sentences: On July 4, Levinson demonstrates that the key to defeating the aliens lies in deactivating their force fields, and devises a way to do so by uploading a computer virus into the mothership. He proposes using the refurbished alien fighter to implement the plan, which Hiller volunteers to pilot.