This is the second book in the 'The Whale Has Wings' series. It carries on directly from the action described at the end of Volume 1 - Rebirth.
While the action described in this book is intended to stand on its own, you may have problems understanding how the Royal Navy (in particular) got here if you haven't read Volume 1. To summarise:
In 1933, the British Royal Navy regained control of its Air Arm (the Fleet Air Arm) from the RAF. This allowed them to build carriers and the aircraft needed for them to their own pre-war plans, rather than be constricted by the limited number and performance of the aircraft grudgingly provided by the RAF. As a consequence, by the time war broke out in 1939, the FAA had better planes, and the building of carriers was a little advanced on our history. In addition, as more planes were available, a class of light carriers was building, and plans had been made for converting merchant ships (in our history, little actual planning for these was done as the RAF would not supply the aircraft required, so priority was very low).
Initially the war doesn't seem to go much differently, until the big raid on the German ships at Wilhelmshaven harbour. Even after this, the diversion isn't yet great, and Norway is still a disaster for the German Navy. However the leverage of the changes in air power are slowly growing. As a result of greater confidence in his carriers, the British are not forced to sink the French ships in Oran, and the attack on the Vichy French at Dakar goes more successfully.
Other changes are less obvious; with more air cover, fewer ships are being lost to aircraft and submarines (as a result, the savings in repair work and new construction more than allows the continuation of the RN carrier building programme). Italy joins the war, and action spreads into the Mediterranean.
This book starts with the British attack on the Italian Fleet at Taranto - this time, the plan is more like the original plans and exercises carried out in preparation for the raid. This books covers the actions through late 1940 and 1941, mainly in the Mediterranean, but also in the Atlantic. As well as the naval actions, details are given of the war on land and air (in particular where this involves British forces), showing how the divergence from our history is slowly increasing.