Precision and good seamanship are essential - minesweeping is still arduous even in peacetime. Moored mines - a flotilla of fleet minesweepers are seen conducting an Oropesa sweep in 'G' formation. Equipment and method of operation. Importance of exact navigation to avoid gaps in the sweep. Laying of danbuoys to mark swept areas. Checksweep in 'J' formation using the 'A' sweep made. Laying and recovery of sweep wires, kites, etc. Influence mines - usually laid in shallow water and therefore swept by motor minesweepers. Magnetic mines - 'LL' and 'LAA' electrical sweeps using wires carrying a pulsed current of 2000-1000A. Acoustic mines - hammer box and explosive sweep. Minesweepers are shown making a combined acoustic and magnetic sweep. Importance of correct station keeping. "Once you have learnt the drill it (minesweeping) is fairly straightforward, but attention to detail is the basis of all minesweeping". Theoretical knowledge obtained at HMS Lochinvar. The film describes in detail a large-scale practice carried out in Largo Bay, Scotland, and involving motor launches, fleet and motor MSD (minesweeper drone). Rôle of research scientists and trials officers in working out sweeps to counter new types of mine. Minesweeping ensures Britain the continued freedom of the seas.