Complex sentence: To defeat them, Kate plans to detonate a nuclear warhead in London from within UNIT's "TARDIS-proof" Black Archive of Time Lord and other artefacts in London which will wipe out London but save the rest of humanity.
Split into simpler sentences: To defeat them, Kate plans to detonate a nuclear warhead in London from within UNIT's "TARDIS-proof" Black Archive of Time Lord and other artefacts. The detonation would will wipe out London but save the rest of humanity.